SQL NULL VALUES

SQL NULL VALUES | IS NULL and IS NOT NULL Operator


SQL NULL value is used to represent a missing value or unknown data in a field. By default, a table field can hold NULL values.

SQL NULL values are treated differently from other data values by sql database engine. Ex: A SQL NULL value is different than a zero value or a column that contains spaces.

SQL NULL value mainly used as a container for unknown data or inapplicable values.



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Sample Database Table - Books

In this below table, the "Description" field in the "Books" table is optional. This means that if we add a new row with no value for the "Description" field, the "Description" field will be stored with a SQL NULL value.

Look at the following "Books" table:

BookId BookName Description
101 Sql Complete Reference It descripes how to write and execute SQL statement into database.
102 Sql Commands
103 Pl Sql Quick Programming How to write and execute SQL programming using pl sql.
104 Sql Query Injection
105 The Power Of Pl Sql
106 Sql Data Analysis It explains how to build and manage SQL database

It is not possible to check for SQL NULL values with comparison operators. If you use any other operators to work with NULL values, the query result will not include actual NULL value records.

So we will have to use the "IS NULL" and "IS NOT NULL" operators when working with NULL values.


SQL "IS NULL" Operator Example

The following sql statement will fetch only the records with NULL values in the "Description" field?


SELECT * FROM Books
WHERE Description IS NULL;

The above query result is:

BookId BookName Description
102 Sql Commands
104 Sql Query Injection
105 The Power Of Pl Sql

SQL "IS NOT NULL" Operator Example

The following sql statement will fetch only the records with no NULL values in the "Description" field?


SELECT * FROM Books
WHERE Description IS NOT NULL;

The above query result is:

BookId BookName Description
101 Sql Complete Reference It descripes how to write and execute SQL statement into database.
103 Pl Sql Quick Programming How to write and execute SQL programming using pl sql.
106 Sql Data Analysis It explains how to build and manage SQL database


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SQL Null Count

Null expressions are not taken account by COUNT(Expression). Count() returns all rows with a non-null value if the column name accepts NULL values.

COUNT(*) returns the total number of rows, containing duplicates, non-NULL rows, and NULL rows. A SELECT statement produced this result set.

Syntax:

count all the NULL values from the table.

SELECT COUNT(Col1,0) CountCol
FROM Table1
WHERE Col1 IS NULL;

When you look at the query result, you'll note that, despite the fact that we have three NULL values, the query indicates there are no NULL values. This is due to the fact that all aggregate values exclude NULL values. If you wish to count the NULL values, you'll need to convert them to different values first, then use the aggregate function, as seen in the accompanying script.

SELECT COUNT(ISNULL(Col1,0)) CountCol
FROM Table1
WHERE Col1 IS NULL;

When you execute the script above, you'll note that the query now shows the right value of NULL values. The purpose for disregarding NULL values is simple: it prevents unknown or inapplicable data from impacting the aggregate result.

Example 1: The following query use COUNT() with IF function to find the number of canceled, on hold and disputed orders from the orders table:

SELECT 
    COUNT(IF(status = 'Cancelled', 1, NULL)) 'Cancelled',
    COUNT(IF(status = 'On Hold', 1, NULL)) 'On Hold',
    COUNT(IF(status = 'Disputed', 1, NULL)) 'Disputed'
FROM
    orders;

If the order's status is cancelled, on hold, or disputed, the IF() function returns 1; otherwise, it returns NULL.

The query returns the number of orders based on the associated status because the COUNT function only counts 1 and not NULL values.

Example 2: Now that we know how to count null, non-null, and all rows in a table, let's see an example.

To display all table rows, the number of populated addressLine2 rows, and Nulls, this query use the Count() function in three ways:

SELECT COUNT(*) AS All_Rows,
  COUNT(addressLine2) AS addressLine2_Count, 
  COUNT(*) - COUNT(addressLine2) AS Null_addressLine2_Rows
FROM customers;

As expected, the addressLine2_Count and Null_addressLine2_Rows results add up to the All_Rows count.


SQL Null Coalesce

When working with NULL, the SQL COALESCE function comes in handy.

SQL COALESCE is useful in this situation. You can use this function to check for NULL and replace it with another value if NULL is found.

If all of the parameters in the MySQL COALESCE() function are assessed as null, this function returns the first non-null parameter in the argument list. This indicates that no non-NULL values are found in the list by this function.

The COALESCE function can be used to replace a NULL value with a default value.

Example 1: Let's start with some simple examples to further clarify how the COALESCE () function works.

SELECT COALESCE (null, 1, 2, 3) FROM dual;

As you can see in the previous instance, we've given the COALESCE () function four parameters, the first of which is NULL, and as previously stated, the COALESCE () function outputs the first 'Not Null' value from the parameter list. As a result, the first 'Not Null' value in the argument list will be 1 when executed.

Example 2:

Select COALESCE(NULL, NULL, NULL, NULL);
COALESCE(NULL, NULL, NULL, NULL)
NULL

Example 3: In this scenario, you can use the COALESCE function to acquire the product summary, or you can get the first 50 characters from the product description if the product summary isn't supplied.

SELECT ID,
product_name,
COALESCE (
product_summary,
	LEFT (product_description, 50)
	) excerpt,
	price,
	discount
FROM
	products;

Example 4: In this example COALESCE(parkalias, parkname) returns the value found in parkalias if the value is not NULL; however, if it is NULL, then parkname is returned.

SELECT parkalias,
parkname,
city,
state
FROM Parks

SQL Count Null as 0

A NULL value signifies an unknown value in the relational database model. A zero value or a field with spaces are not the same as an unknown value.

Use count(*), which works in the same way as count(1) or any other constant.

The * will ensure every row is counted.

Select Group, Count(*) 
From [Data] 
Group by [Group]

Because each cell is counted as either 1 or null, and null + null = null, the sum of that group would also be null, null displays 0 instead of 2. However, because the column type is an integer, it appears as 0.


SQL Null

To determine if a value in a column is NULL or not, SQL uses the IS NULL or IS NOT NULL functions. In a table, a NULL value is a value in a field that seems to be empty.

When a NULL is used in a comparison operation, the result is treated as UNKNOWN. As a result, SQL employs three-valued logic: True, False, and Unknown. When the logical connectives AND, OR, and NOT are utilised, it is important to specify the consequences of three-valued logical expressions.

A NULL value is used to denote a missing value, however it can be interpreted in one of three ways:

  • The amount is unknown (value exists but is not known), A field that has a NULL value has no value.
  • It's critical to distinguish between a NULL value and a zero value or a field with spaces.
  • No value is given (exists but is purposely withheld)
  • Not related attribute (undefined for this tuple)

Note: Traditional comparison operators (=,<, >, and <>) cannot be used in queries due to the structure of NULL values. To return empty result sets, use the WHERE clause.

SELECT column_name1, column_name2, column_name3, ... , column_nameN
FROM table_name
WHERE column_nameN = NULL

As a result, working with NULL values can be challenging and requires the usage of built-in functions that are specifically designed to handle NULL values.

Principles of NULL values:

  • When the real value is unknown or a value would be meaningless, a NULL value is fine.
  • If the data type is a number, a NULL value is not comparable to ZERO, and if the data type is a character, it is not similar to spaces.
  • A NULL value can be placed into any data type column.
  • In any expression, a NULL value will evaluate to NULL.
  • If any column has a NULL value, SQL will ignore UNIQUE, FOREIGN key, and CHECK restrictions.

Syntax:

SELECT [column_name1,column_name2 ]
FROM [table_name]
WHERE [column_name] IS [NOT] NULL; 

The basic syntax of NULL while creating a table.

CREATE TABLE CUSTOMERS(
   ID   INT              NOT NULL,
   NAME VARCHAR (20)     NOT NULL,
   AGE  INT              NOT NULL,
   ADDRESS  CHAR (25) ,
   SALARY   DECIMAL (18, 2),       
   PRIMARY KEY (ID)
);

A NULL value indicates that a field was left blank during the record creation process.

Example 1: Consider the following CUSTOMERS table having the records as shown below.

ID NAME AGE ADDRESS SALARY
1 Ramesh 32 Ahmedabad 2000.00
2 Khilan 25 Delhi 1500.00
3 kaushik 23 Kota 2000.00
4 Chaitali 25 Mumbai 6500.00
5 Hardik 27 Bhopal 8500.00
6 Komal 22 MP
7 Muffy 24 Indore

Now, following is the usage of the IS NULL operator.

SELECT  ID, NAME, AGE, ADDRESS, SALARY
FROM CUSTOMERS
WHERE SALARY IS NULL;

Output:

ID NAME AGE ADDRESS SALARY
6 Komal 22 MP
7 Muffy 24 Indore

Example 2:

SELECT Fname, Lname FROM Employee WHERE Super_ssn IS NULL;

Example 3: Using the following criteria, get data from all columns in the 'listofitem' table :

1. coname column contain NULL value, the following sql statement can be used :

SELECT *  
FROM listofitem  
WHERE coname IS NULL;

SQL Null and Not Null

In the where clause, employ comparison operators like =, <, >, etc. However, if a table column (field) contains null values, such operators will not operate; in this situation, we must use the IS NULL and IS NOT NULL operators to verify for null values.

NULL data denotes data that does not exist, is missing, or is unknown. IS NULL and IS NOT NULL can be used in the same SQL query in the WHERE clause in any order and in any combination as needed, such as SELECT, UPDATE, and DELETE statements/queries, to evaluate whether a column has data or not. Please keep in mind that NULL and 0 are not synonymous. A column with a NULL value is empty and has no value.

The check requirements NULL and NOT NULL are used to declare whether a column should permit nulls or not. This can be specified during the creation of database objects. That is, whether the column can take a NULL value or whether it must have a value.

NULL denotes an unknown, null, or missing value.

The equal operator (=) is used to check the condition with NULL.

Eg: When user leaves the field without entering details.

NOT NULL - non empty value.

The Not Null function is used to determine whether or not a value is null. The condition becomes successful if the value is not null.

Eg: When user fill the field with data.

Syntax for SQL IS NULL

SELECT column_name1, column_name2, etc
FROM table_name
WHERE  column_name1 IS NULL;

Syntax for SQL IS NOT NULL

SELECT column_name1, column_name2, etc
FROM table_name
WHERE  column_name1 IS NOT NULL;

EXAMPLE 1: HOW TO USE IS NULL IN WHERE CLAUSE IN SELECT QUERIES:

SELECT Student_ID, Student_name, City, Age from student
WHERE Age IS NULL;

This SQL only selects records with a NULL Age value. The rest of the records aren't chosen.

HOW TO USE IS NOT NULL IN WHERE CLAUSE IN SELECT QUERIES:

SELECT Student_ID, Student_name, City, Age from student
WHERE Age IS NOT NULL;

This SQL only selects entries with an Age value that is not NULL. The rest of the records aren't chosen.

Example 2: Let's see an example below.

Step 1:

DROP TABLE IF EXISTS temp.dbo.#tempFruits;

Step 2:

CREATE TABLE #tempFruits
(Id INT IDENTITY,
 [Name] NVARCHAR(10) NOT NULL,
 [Description] NVARCHAR(50) NULL);

Step 3:

INSERT INTO #tempFruits (Name, Description)
 VALUES
 ('Apple', 'The domesticated tree Malus'),
 ('Kiwifruit', NULL),
 ('Pineapple', NULL),
 ('Papaya', 'Also called papaw or pawpaw'),
 ('Cherry', NULL)

Step 4:

SELECT * FROM #tempFruits WHERE [Description] = NULL -- 0 result
 SELECT * FROM #tempFruits WHERE [Description] IS NULL -- 3 results

 SELECT * FROM #tempFruits WHERE [Description] <> NULL -- 0 result
 SELECT * FROM #tempFruits WHERE [Description] IS NOT NULL -- 2 results

As you can see, we've coupled the equal and not operators with their NULL equivalents in our code instance above.

To check whether a column is NULL, we utilised the IS NULL operator rather than the = operator.

SELECT * FROM #tempFruits WHERE [Description] = NULL -- 0 result
SELECT * FROM #tempFruits WHERE [Description] IS NULL -- 3 results

Second, instead of using the <> operator to check whether a column is NULL, we utilised the IS NOT NULL operator.

SELECT * FROM #tempFruits WHERE [Description] <> NULL -- 0 result
SELECT * FROM #tempFruits WHERE [Description] IS NOT NULL -- 2 results

Example 3: SQL IS NULL or empty check Example

Table: Employees

EmployeeName EmployeeAge EmployeePhoneNo EmployeeAddress
Cindi 34 95XXXXXXX8 Noida
Linda 35 Agra
Shaun 33 75XXXXXXX4 Delhi
Timmy 34 Noida
Pappu 36 Noida

The EmployeeName and EmployeeAddress data of employees with a null EmployeePhoneNo column will be retrieved using the SQL statement below.

SELECT EmployeeName, EmployeeAddress
FROM Employees
WHERE EmployeePhoneNo IS NULL;

Result:

EmployeeName EmployeeAddress
Linda Agra
Timmy Noida
Pappu Noida

SQL IS NOT NULL in where clause Example

We'll look for not null values in this case.

The EmployeeName and EmployeeAddress data of employees whose EmployeePhoneNo field is not null will be retrieved using the SQL statement below.

SELECT EmployeeName, EmployeeAddress
FROM Employees
WHERE EmployeePhoneNo IS NOT NULL;

Result:

EmployeeName EmployeeAddress
Cindi Noida
Shaun Delhi

SQL Null Comparison

The SQL NULL value has a specific function. It also has certain odd characteristics that can confuse SQL newbies and even expert programmers. To prevent these issues, we can use NULL with comparison operators.

SQL allows users to create search criteria that comprise a number of predicates, each of which evaluates rows using a distinct operator.

Equal operators cannot match NULL to any other value. So, since we can't utilise any operator or mathematical operation, how can we use NULL? To check and test values versus NULL, PostgreSQL provides special statements and functions. In PostgreSQL, there is only one way to utilise NULL.

Use the negated comparison => (null-safe equality operator), which returns FALSE if one of the operands is null but TRUE if both are null and both operands have equal non-null values.

Example 1: Run the following queries, which are identical to the previous example but utilise a different comparison operator to demonstrate how the other comparison operators function.

Because the <> operator checks whether two values are similar, this query returns every record with a goal value less than 40:

SELECT name, goal
FROM running_goals
WHERE goal <> 40;

Output:

Name Goal
Michelle 55
Jerry 25
Milton 45
Wanda 30
Stewart 35

Example 2: IS NULL/IS NOT NULL

NULL Comparison UsingShell

postgres=# SELECT NULL is NULL result;
Result
t

Example 3: query using comparison and logical operators. Guess how many rows it returns:

SELECT spouse
  FROM simpsons
 WHERE      spouse = NULL
    OR NOT (spouse = NULL)

Whatever the comparison column includes—salary, pet names, etc.—the answer is unknown if we verify that it is equivalent to NULL. Even if the column value is NULL, this is true. This is what perplexes programmers with prior programming experience.


SQL Null Date to Blank

When a datetime column is NULL, I need to convert it to varchar so that I can display a blank or a message.

Example: This is what I have so far:

select
isnull(us.Date,0) as USDate,
isnull(au.Date,0) as AUDate
from ustable us
left join autable au
on us.column=au.column

Output:

USDATE
2014-10-24 10:29:07.450
1900-01-01 00:00:00.000

I would like to be able to make the "1900-01-01 00:00:00.000" varchar so I can write a message or show a true blank.


SQL Null Default

Allowing nulls by default to true switches a column's default null type from not null to null, as required by the SQL standard. The default value for a column in Transact-SQL is not null, which means that null values are not permitted unless null is mentioned in the build table or change table column specification.

Permit nulls by default cannot be used to adjust the nullability of a column in select into statements. Alternatively, define the nullability of the generated columns with convert().

If the column has no value supplied in the INSERT statement, the DEFAULT value constraint occurs. By explicitly adding NULL into an optional (nullable) column, you can still insert a NULL.

Example 1: In the instance of a table with three columns, two of them contain default values. One of the columns is nullable, whereas the other is not.

Step 1:

CREATE TABLE TestTable 
        (ID INT, 
        NotNullCol INT NOT NULL DEFAULT 1, 
        NullCol INT NULL DEFAULT 1)
GO

Step 2:

INSERT INTO TestTable (ID)
VALUES (1)
GO

Step 3:

SELECT * FROM TestTable
GO

Step 4:

DROP Table TestTable
GO

We added the data into the table after it was created.

Now choose the data from the table, and the following resultset will appear.

The result set clearly shows that both columns with default values have default values when we insert the data.

Example 2:

INSERT INTO foo VALUES (1, NULL);

SQL Null Delete

Check for null values in that column with the Null operator, then delete them with the DELETE command.

Syntax:

Here is the SQL query to delete null values in SQL:

DELETE FROM Table_name
WHERE column_name  = ‘   ’

or

DELETE FROM Table_name
WHERE column_name  IS NULL

Syntax for mysql on delete set null

FOREIGN KEY (child_col1, child_col2, ... child_col_n)
 REFERENCES parent_table (parent_col1, parent_col2, ... parent_col_n)
 ON DELETE SET NULL
 [ ON UPDATE { NO ACTION | CASCADE | SET NULL | SET DEFAULT } ]

Example: query to delete blank rows as well as NULL:

delete from deleteRowDemo where StudentName='' OR StudentName IS NULL;

Now let us check the table records once again.

select *from deleteRowDemo;

Output:

Id StudentName
1 John
5 Carol
6 Bob
8 David

SQL Null Divide by Number

Example 1: To calculate the unitPrice of the product:

Product table in SQL which has columns Product ID, TotalPrice, NumberOfUnits

This can be calculated by dividing the TotalPrice by NumberOfUnits.

if the totalPrice is Null and NumberOfUnits is 45.

Select 
 ProductID,
 (TotalPrice / NumberOfUnits) as UnitPrice 
from ProductTable

Example 2: NULL divided by any Number:

SELECT NULL / 45

Result:

Column1
NULL

SQL Null Function

The SQL Server ISNULL Function is used to replace NULL values in SQL with a particular value.

IFNULL, COALESCE, and NULLIF are three useful MySQL functions that easily manage NULL.

Syntax:

The syntax for the SQL ISNULL function is as follow.

ISNULL (expression, replacement)
  • Expression: The expression in which we need to check NULL values is specified in this argument.
  • Replacement: We want to replace NULL with anything particular. Here we must supply a replacement value.

If the first parameter expression evaluates to NULL, the ISNULL function in SQL Server returns the replacement value. SQL Server translates the replacement data type to the expression data type.

Example 1: The following statement returns the phone number if it is not NULL; alternatively, it returns N/A instead of NULL.

SELECT id, 
  first_name, 
  last_name, 
  IFNULL(phone, 'N/A') phone
FROM
  leads;

Example 2: Because the first parameter is NULL, this example utilises the ISNULL() function to return the second argument:

SELECT ISNULL(NULL,20) result;

Output:

Result
20

Example 3: Consider the following Employee table:

Find the sum of salary of all Employee, if Salary of any employee is not available (or NULL value), use salary as 10000.

SELECT SUM(ISNULL(Salary, 10000) AS Salary
FROM Employee;

Example 4: SQL Server ISNULL function in an argument

In this example, SQL ISNULL function returns the second argument value because the first argument is NULL:

SELECT ISNULL(NULL, 100) result;

SQL Null Groupby

The result of comparing two separate NULL values is NULL (not TRUE), indicating that the two NULL values are not identical.

However, because generating a new group for each result with a NULL in a grouping column is confusing and useless, SQL designers wrote the standard such that NULL values are treated the same when using the GROUP BY clause. The DBMS will group two rows together if they have NULL values in the same grouping columns and matching values in the additional non-NULL grouping columns.

In the results, if the grouping column is null, that row gets its own group. If there are multiple null values in the grouping column, the null values form a cluster.

Example 1: For example, the grouped query:

SELECT A, B, SUM(amount_purchased) AS 'C'
FROM customers
GROUP BY A, B
ORDER BY A, B

will display a results table similar to

A B C
NULL NULL 61438.0000
NULL 101 196156.0000
AZ NULL 75815.0000
AZ 103 36958.0000
CA 101 78252.0000
LA NULL 181632.0000

for CUSTOMERS that contain the following rows.

A B Amount_purchased
NULL NULL 45612.00000
NULL NULL 15826.00000
NULL 101 45852.0000
NULL 101 74815.0000
NULL 101 75489.0000
AZ NULL 75815.0000
AZ 103 36958.0000
CA 101 78252.0000
LA NULL 96385.0000
LA NULL 85247.0000

Example 2: This example uses group by and the advance column, which contains some null values:

select advance, avg(price * 2) 
from titles 
group by advance

Output:

Advance
NULL NULL
0.00 39.98
2000.00 39.98
2275.00 21.90
4000.00 19.94
5000.00 34.62
6000.00 14.00
7000.00 43.66
8000.00 34.99
10125.00 5.98
15000.00 5.98

Example 3: Our base table had no null values in the samples used so far in the lesson. Now let's look at how to group a table with null values.

Now consider the following GROUP BY query.

select grp_a, grp_b, count(*)
from   t2
group by grp_a, grp_b
order by grp_a, grp_b ;

Output:

GRP_A GRP_B COUNT(*)
A1 X1 1
A1 X2 1
A1 (null) 3
A2 (null) 1

SQL Null Insert

A NULL value can be inserted into a column using the SQL INSERT statement.

If you need to insert rows into tables with NULL values. No data is represented by the term NULL(without quotes).

A NULL value is used to signify something other than zero or missing data, and it usually has one of three interpretations:

  • The value is unknown (value exists but is not known)
  • No value is available (exists but is purposely withheld)
  • Not relevant attribute (undefined for this tuple)

Syntax:

INSERT INTO TABLE_NAME values
(COLUMN_VALUE,NULL,........);

Example 1: Insert 10 rows into the WORKER table.

INSERT INTO WORKER VALUES('SAM','ONTARIO',NULL);
INSERT INTO WORKER VALUES('TIM',NULL,56);
INSERT INTO WORKER VALUES(NULL,'CAIRO',43);
INSERT INTO WORKER VALUES(NULL,'MUMBAI',NULL);
INSERT INTO WORKER VALUES(NULL,NULL,NULL);

Example 2: To add values'A001','Jodi','London','.12','NULL' for a single row into the table 'agents' then, the following SQL statement can be used:

INSERT INTO agents
VALUES ("A001","Jodi","London",.12,NULL);

Example 3:

INSERT INTO MyTable (Col1, Col2, Col3)
VALUES (1, NULL, 3)

or

INSERT INTO MyTable (Col1, Col2, Col3)
SELECT 1, NULL, 3

SQL Null in Where

IS NULL & IS NOT NULL in SQL are commonly used in SELECT, UPDATE, and DELETE statements/queries with a WHERE clause to check whether a column has data or not.

Example 1: USE IS NULL IN WHERE CLAUSE IN SELECT QUERIES:

SELECT Student_ID, Student_name, City, Age from student
WHERE Age IS NULL;

This SQL selects only the records whose Age value is NULL. The remaining records are not selected.

Example 2: SQL IS NOT NULL in where clause Example

Lets take the same example that we have seen above. In this example, we will check for the not null values.

Table: Employees

EmployeeName EmployeeAge EmployeePhoneNo EmployeeAddress
Cindi 34 95XXXXXXX8 Noida
Linda 35 Agra
Shaun 33 75XXXXXXX4 Delhi
Timmy 34 Noida
Pappu 36 Noida

The EmployeeName and EmployeeAddress data of employees whose EmployeePhoneNo field is not null will be retrieved using the SQL statement below.

SELECT EmployeeName, EmployeeAddress
FROM Employees
WHERE EmployeePhoneNo IS NOT NULL;

Result:

EmployeeName EmployeeAddress
Cindi Noida
Shaun Delhi

SQL Null or Empty

Main Article :- Sql difference between NULL and EMPTY Values

NULL is a reserved keyword in SQL that represents values that are absent or unknown. Null is a state, not a value; it does not represent 0 or an empty string.

In SQL Server, the functions ISNULL() and COALESCE can be used to replace NULL with blank data. If the parameter is NULL, both methods will return an empty string. For example, ISNULL (column, "") will return an empty string if the column value is NULL.

In SQL Server, use NULL and an empty string in the same WHERE clause. If I need to find records with null values or an empty string, I'll use this method.

Example 1: SQL Query to Select All If Parameter is Empty or NULL

Let's look at an example of how to build a SQL query to select all if a parameter is empty or NULL.

We used the IIF function in conjunction with ISNULL in this instance. The ISNULL function first determines whether or not the given value is NULL. If true, the value will be replaced with an empty string or blank. IIF will then determine whether or not the parameter is blank. If true, occupation equals (=) occupation; alternatively, occupation equals (=) the result provided by the user.

DECLARE @Occupation VARCHAR(50)
SET @Occupation = 'Management'
SELECT [EmpID]
  ,[FirstName]
  ,[LastName]
  ,[Education]
  ,[Occupation]
  ,[YearlyIncome]
  ,[Sales]
  ,[HireDate]
  FROM [Employee]
WHERE Occupation = IIF(ISNULL(@Occupation, '') = '', Occupation, @Occupation)

Example 2: The terms "NULL value" and "empty string" are not comparable. I'm curious if NULL can be interpreted as ". The only other option I can think of is to replace NULL with " after you have the data in a csv file.

SELECT ct.ID, 
  ISNULL(NULLIF(ct.LaunchDate, ''), null) [LaunchDate]
FROM   [dbo].[CustomerTable] ct

Example 3: Query to get records without null or empty values:

In SQL Server, run the following query to remove null or empty values from a datatable. To do so, create a table in the database called UserInfo and populate it with the information given below.

SELECT UserId,Name,City,Education FROM UserInfo WHERE City IS NOT NULL AND City <>''

SQL Null or Empty String Check

Any datatype can have a NULL value in SQL. This is not the same as a zero integer or a blank string.

Example 1: sql check for null and empty string

select * from vendor
where isnull(vendor_email,'') = ''

Example 2: retrieving all records where the supplier_name contains a null value:

select * from suppliers
where supplier_name is null;

Both rows will be returned if you execute this statement. This is because Oracle's rules have changed to treat empty strings as null values.

It's also worth noting that the null value is unusual in that it can't be used with the typical operands (=,<, >, etc). Instead, the IS NULL and IS NOT NULL criteria must be used.

Example 3:

SELECT 
  ISNULL(NULLIF(listing.Offer_Text, ''), company.Offer_Text) AS Offer_Text
FROM table;

SQL Null to 0

If you want to convert null values to zeros, use the IFNULL or COALESCE() functions. To avoid divide-by-zero mistakes, use NULLIF.

Use IsNull to substitute a potentially null column with something else. If myColumn is null in the first place, this will set it to 0. COALESCE() and ISNULL() are compared: The goal of the ISNULL function and the COALESCE expression are similar, although they can act differently.

Use sql expressions in which one of the values is null and the other is added or multiplied. When you multiply or add a non-null value to a null value in SQL, you'll get a null answer. Insert sql statements to guarantee null values are always transformed to zero values, resulting in a non-null answer when that is the correct outcome.

Syntax:

SELECT IFNULL(yourColumnName,0) AS anyAliasName FROM yourTableName;

The second syntax is as follows:

SELECT COALESCE(yourColumnName,0) AS anyAliasName FROM yourTableName;

Example 1: Here is example sql from an employee percent allocation report:

ISNULL( 100* (SELECT SUM(te.workhrs) FROM t_timeentry as te    
WHERE te.strt>=$P{startDate}    
AND 
te.strt<$P{finishDate}    
AND 
te.userid=t_timeentry.userid) / NULLIF( (SELECT SUM(te.hours) FROM t_timeentry as te    
WHERE te.strt>=$P{startDate}    
AND 
te.strt<$P{finishDate}    
AND
te.userid=t_timeentry.userid) ,0) ,0)

Exmaple 2: converting NULL to 0

UPDATE fyi_links SET counts=ISNULL(counts,0);
GO
SELECT id, counts FROM fyi_links;
GO

Output:

Id Counts
101 0
102 8
1101 0
202 0
2101 0
2102 0
301 0
302 0

Example 3:

Step 1: Let us first create a table. The query to create a table is as follows

create table convertNullToZeroDemo
 (
  Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
  Name varchar(20),
  Salary int
 );
Query OK, 0 rows affected (1.28 sec)

Step 2: Insert some records in the table using insert command.

insert into convertNullToZeroDemo(Name,Salary) values('John',NULL);
Query OK, 1 row affected (0.20 sec)

insert into convertNullToZeroDemo(Name,Salary) values('Carol',5610);
Query OK, 1 row affected (0.10 sec)

insert into convertNullToZeroDemo(Name,Salary) values('Bob',NULL);
Query OK, 1 row affected (0.15 sec)

insert into convertNullToZeroDemo(Name,Salary) values('David',NULL);
Query OK, 1 row affected (0.12 sec)

Step 3: Display all records from the table using select statement.

select *from convertNullToZeroDemo;

Output:

Id Name Salary
1 John NULL
2 Carol 5610
3 Bob NULL
4 David NULL

Step 4: Here is your first query to convert MySQL NULL to 0

select IFNULL(Salary,0) as `CONVERT_NULL _TO_0` from convertNullToZeroDemo;

Output:

CONVERT_NULL _TO_0
0
5610
0
0

Step 5: Here is your second query to convert MySQL NULL to 0

select coalesce(Salary,0) as `CONVERT_NULL _TO_0` from convertNullToZeroDemo;

Output:

CONVERT_NULL _TO_0
0
5610
0
0

SQL Null with Orderby

For a long time, the NULLS LAST option for the ORDER BY clause has been an ANSI standard. Obviously, this does not imply that every database vendor has adopted it. You're doomed to fail if you're seeking for it in SQL Server or MySQL. Only PostgreSQL and Oracle developers are fortunate in this regard. ORDER BY with NULLS FIRST/LAST is supported in the two databases.

NULL values are larger than non-NULL values in PostgreSQL. All NULL values will be shown last in the output if you sort it in ascending order with the ASC keyword or by default (i.e. not specifying the order).

NULL values are treated as the lowest values by SQL Server.

Example 1: Query with ORDER BY column DESC will display nulls last (at the bottom):

SELECT product, number FROM products
ORDER BY number DESC;

Output:

Product Number
Eggs 7
Butter 3
Cheese 2
Milk 2
Bacon 0
Bread NULL
Sausage NULL

Example 2: Here’s an example:

SELECT *
FROM paintings
ORDER BY year;
Id painting author year
4 The Night Watch Rembrandt 1642
2 The Starry Night Vincent van Gogh 1889
3 The Scream Edvard Munch 1893
1 Mona Lisa Leonardo da Vinci NULL
5 The Birth of Venus Botticelli NULL

You'll obtain NULL values at the top of the result table if you use the DESC keyword in ORDER BY to sort items in decreasing order.

SELECT *
FROM paintings
ORDER BY DESC year;

Output:

Id painting author year
1 MonaLisa Leonardo da Vinci NULL
5 The Birth of Venus Sandro Botticelli NULL
3 The Scream Edvard Munch 1893
2 The Starry Night Vincent van Gogh 1889
4 The Night Watch Rembrandt 1642

Example 3: For example when sorted in ascending order, NULLs come first.

SELECT value
FROM test
ORDER BY value ASC

SQL Null Update

In SQL, null values can be replaced by searching a column in a table for nulls and replacing them with UPDATE, SET, and WHERE. They are ignored in aggregation functions, so make sure this is the behavior you desire; alternatively, you must replace null values with appropriate values.

When no value is submitted by the user or no value exists, Edit fields with NULL values are used. To utilize NULL values in columns, your database administrator must allow them. When you modify values to NULL, SQL throws an error if NULL values are not permitted.

For instance, if you discover that information is wrong, you may want to reset values and ask users to reenter it. You could inform the user by setting table column values to NULL.

Example 1: The following UPDATE statement sets the user's first name to NULL:

UPDATE Customer
SET first_name = NULL
WHERE CustomerId = 123 

Example 2: Setting a field to NULL works exactly like with any other value:

UPDATE Employees 
SET ManagerId = NULL
WHERE Id = 4