SQL Exists Operator

The SQL EXISTS operator is used to test for the existence of any rows or records in a subquery. It can be used with any valid sql statement.

It is used to check whether the result of a sub query or inner query is empty (contains no tuples or records) or not. It returns true if the subquery returns at least one row.

The subquery will be any valid sql SELECT statement.

Note: It is not recommended to use on large result set due to inefficient since the sub-query is return for every row or record in the outer query table.

• The subquery must be enclosed in (brackets).
• You must create alias name for table to access column names (in the where condition) of inner table or subquery table. Bydefault the column will take as outer table column.

• You cannot include columns from subquery table to final output result set.
• You should refer table alias name for subquery table columns while accessing them in the where condion.

ANY and EXISTS two queries are quite different.

The EXISTS operator is a logical operator that lets you see if a subquery produces any results. If the subquery produces one or more rows, the EXISTS operator returns TRUE.

Exists is the same as any other, except that the time spent will be less because in ANY, the query continues to execute until the condition is met and returns results. If it exists, it must first search the entire database for any records that match, and then run it.

The ANY operator is a logical operator that compares a scalar value to a single-column set of values returned by a subquery.

When one or more records satisfy the sub condition and the result is true for those records, any is used.

Example :

Based on whether the subquery returns any rows or none, the first query returns all rows or none.

select code from account where exists (select 1 from store where store.account = account.code)

Syntax:

insert but with a precheck to see if the key of my new inserted record already exists in the target table.

INSERT target (field1, field2, field3,.....)
SELECT field1, field2, field3
FROM Source
WHERE NOT EXISTS (SELECT 1 FROM target t2 WHERE t2.field1 = Source.field1)

Example: sql check if there is an entry exists before insert

IF NOT EXISTS (SELECT * FROM EmailsRecebidos 
   WHERE De = @_DE
   AND Assunto = @_ASSUNTO
   AND Data = @_DATA)
 BEGIN
  INSERT INTO EmailsRecebidos (De, Assunto, Data)
  VALUES (@_DE, @_ASSUNTO, @_DATA)
  END

The INSERT IF NOT EXISTS clause in MySQL assists us in performing this action quickly.

Insert records and see whether they already exist. In SQL, the EXISTS condition is used to determine whether or not the result of an associated nested query is empty (i.e., includes no tuples). It can be used to insert, select, update, or delete data in a database.

Then, without verifying their existence in the table, they frequently enter certain values in a particular column. However, there are situations when it is vital to avoid entering duplicate entries into a table.

Syntax:

The basic syntax for INSERT IF NOT EXISTS is as follows.

INSERT INTO name_of_the_table (column_name)
SELECT * FROM (SELECT value_name) AS val
WHERE NOT EXISTS (<conditonal expression>);

In the name_of_the_table we insert the value_name in the column_name if the conditional expression is met.

Example 1: insert data into my table, but insert only data that doesn't already exist in my database.

ALTER PROCEDURE [dbo].[EmailsRecebidosInsert]
 (@_DE nvarchar(50),
 @_ASSUNTO nvarchar(50),
 @_DATA nvarchar(30) )
AS
BEGIN
 INSERT INTO EmailsRecebidos (De, Assunto, Data)
 VALUES (@_DE, @_ASSUNTO, @_DATA)
 WHERE NOT EXISTS ( SELECT * FROM EmailsRecebidos 
  WHERE De = @_DE
  AND Assunto = @_ASSUNTO
  AND Data = @_DATA);
END

And the error is:

Example 2: Sql Server check table exists before creating

The OBJECT ID is used in this example to determine whether or not a table exists in SQL Server. Based on the condition outcome, we utilised the IF ELSE expression to print alternative outputs (Message).

IF OBJECT_ID('dbo.Employees', 'U') IS NOT NULL 
BEGIN
PRINT 'Table Exists in SQL Test Database'
END
ELSE
BEGIN
PRINT 'Table Does not Exists'
END

To check if data exists in a table and when to use it to enhance query efficiency, use the MySQL EXISTS operator.

In SQL Server, how can I see if a record exists in a table:

• To check if a record exists, use the EXISTS clause in the IF statement.
• To check if a record exists, use the EXISTS clause in the CASE statement.
• To check if a record exists, use the EXISTS clause in the WHERE clause.

Example 1: The following statement returns TRUE if there is a customer whose total order price is less than 200:

SELECT Name
FROM Clients
WHERE EXISTS (
 SELECT OrderNbr
 FROM Orders
 WHERE Orders.ClientID = Clients.ClientID 
 AND Total < 200
);

Output:

Example 2: sql server if exists update else insert:

if exists(SELECT * from Student where FirstName='Akhil' and LastName='Mittal')            
BEGIN            
 update Student set FirstName='Anu' where FirstName='Akhil'  
End                    
else            
begin  
insert into Student values(1,'Akhil','Mittal',28,'Male',2006,'Noida','Tenth','LFS','Delhi')  
end 

The SQL DROP statement has an optional IF EXISTS clause that can be used as DROP IF EXISTS. After ensuring that the table exists in the database, this is the simplest approach to remove it.

The advantage of this phrase is that we can only drop a table if it is present in the database. Essentially, it saves writing an if condition and a statement to check the existence of the object within the if condition.

EXIT IF DROP If an item already exists, drop it and recreate it; otherwise, continue performing the provided code.

Syntax:

DROP OBJECT_TYPE [ IF EXISTS ] OBJECT_NAME

WHERE: OBJECT_TYPE like Table, Procedure, View, Function, Database, Trigger, Assembly, Sequence, Index etc.

IF EXISTS: It is an optional phrase that, if included in the DROP statement, checks for the existence of the object; if it exists, it is dropped; otherwise, the following statement in the block is executed without issue.

Example 1: Let's look at some samples to help us comprehend the new IF EXISTS clause in the DROP statement. Let's start by creating a sample database with a table and a stored procedure to illustrate this clause.

DROP PROCEDURE IF EXISTS dbo.WelcomeMessage

If the stored procedure does not exist, no error will be raised; instead, the batch will continue to execute the following statement. Let's attempt re-dropping the previously dropped stored procedure WelcomeMessage.

Example 2:

DROP TABLE IF EXISTS dbo.SampleTable;
GO

CREATE TABLE dbo.SampleTable
(
Id INT,
Name VARCHAR(50)
);
GO

When the SQL EXISTS condition is used with a subquery, it is evaluated to be met if the subquery returns at least one row. It can be used in a SELECT, INSERT, UPDATE, or DELETE statement.

If the subquery produces one or more records, EXISTS returns true.

The EXISTS subquery checks if a subquery retrieves at least one row. This operation returns 'FALSE' if no data is returned.

A valid EXISTS subquery must have at least one outside reference and be correlated.

WHERE EXISTS determines whether or not a subquery returns any results.

The EXISTS subquery's choose list isn't used to evaluate the EXISTS, therefore it can include any valid select list.

In most cases, we utilise the EXISTS operator in the WHERE clause to see if the subquery is yielding any results.

In general, use EXISTS when:

• Data from the associated table is not required to be returned.
• In the associated table, you have duplicates (JOIN can cause duplicate rows if values are repeated)
• You want to make sure it exists (use instead of LEFT OUTER JOIN...NULL condition)
• If you have good indexes, the EXISTS will almost always perform the same as the JOIN. With the exception of particularly intricate subqueries, where EXISTS is usually faster.

Syntax:

EXISTS syntax below:

SELECT column-names
 FROM table-name
 WHERE EXISTS 
 (SELECT column-name
  FROM table-name
 WHERE condition)

Example 1: Let's look at how to use the Exists Operator in SQL Server using a SELECT Statement. The EXISTS operator is combined with the Select statement in the example below. Here, the EXISTS condition will verify and return any records from the EmployeeDetails table that have at least one matching employee ID in the EmployeeContactDetails table.

SELECT *
FROM EmployeeDetails
WHERE EXISTS (SELECT *
 FROM EmployeeContactDetails
 WHERE EmployeeDetails.ID = EmployeeContactDetails.EmployeeID);

Example 2: List customers with orders over $5000.

SELECT *
 FROM Customer
 WHERE EXISTS
 (SELECT Id
 FROM [Order]
 WHERE CustomerId = Customer.Id 
 AND TotalAmount > 5000)	

Note: This is a subquery that is linked to Customer. The inner query's id refers to the outer query.

IF EXISTS(SELECT * FROM sys.columns
   WHERE Name = 'columnName' 
   AND OBJECT_ID = OBJECT_ID('tableName') )
BEGIN
   PRINT 'Your Column Exists'
END

I want to know how to convert this code that it can check multiple column name existence.

The SQL EXISTS clause is almost always quicker than COUNT(*) for checking if something exists. COUNT(*) must count every row, even if it knows one row has passed the test, because EXISTS can terminate as soon as the logical test proves true.

COUNT will count the number of non-null values returned if an expression is supplied. So, even if the table contains 0 rows, this will always evaluate to true.

This will not produce the desired results. WHERE EXISTS () can be written with EXISTS, although count(*) > 0 has an implicit subquery. Only a HAVING condition could use the literal form count(*) > 0. Without an outer connect, this isn't really useful.

Syntax:

IF EXISTS(SELECT COUNT(1) FROM myTable)

Example 1: A search for the brand and size of pet food in stock at any of our pet shops.

With EXISTS:

SELECT brand, size 
 FROM pet_food 
 WHERE EXISTS (SELECT * FROM store_inventory 
 WHERE store_inventory.code = pet_food.code); 

With a left join, we can use HAVING:

SELECT brand, size 
  FROM pet_food LEFT JOIN store_inventory USING (code) 
GROUP BY brand, size 
  HAVING count(*) > 0;

Example 2:

select case
  when exists (select *
  from   customer
 where  amount <> 0
 and customerid = 22) then 1
 else 0
 end  as non_zero_exists

Example 3: Run the following SELECT statement to create a test table based on the OrderDetails table from Northwind:

SELECT * INTO tempdb..OrderDetails FROM \[Northwind\]

I didn't make any indexes on this table to keep the sample basic. Now execute the following code to see if a specific value in a table exists using EXISTS:

SET STATISTICS IO ON
IF EXISTS(SELECT * FROM OrderDetails
WHERE orderid = 10248)
PRINT 'yes'
ELSE
PRINT 'no'
SET STATISTICS IO OFF

The code passes the test, giving you the following STATISTICS IO information:

Table 'OrderDetails'. Scan count 1, logical reads 2, physical reads 0, read-ahead reads 0.

Now run the following code, which uses COUNT(*) to check whether a particular value exists in a table:

SET STATISTICS IO ON
IF (SELECT COUNT(*) FROM OrderDetails
WHERE orderid = 10248) > 0
PRINT 'yes'
ELSE
PRINT 'no'
SET STATISTICS IO OFF

This code also passes the test. However, you'll see the following STATISTICS IO information:

Table 'OrderDetails'. Scan count 1, logical reads 10, physical reads 0, read-ahead reads 0.

The EXISTS check detected the entry immediately away in this basic example on a tiny table, lowering the search to two logical reads. Ten logical reads were completed by the COUNT(*) check. The difference in efficiency between an EXISTS check and COUNT(*) is often even more obvious.

If the result of the subquery does not have any rows, both EXISTS and NOT EXISTS return TRUE. NOT EXISTS allows you to locate records that don't match the subquery in plain English.

The logical operator SQL EXISTS is used to examine if records in a database exist. The database engine isn't required to conduct the subquery in its entirety. If the subquery produces at least one record, the EXISTS operation returns true, otherwise false. However, the WHERE clause employs the EXISTS operator in conjunction with an inner subquery.

The EXISTS condition returns a boolean value of True or False. EXISTS can be used in SQL Server SELECT, UPDATE, INSERT and DELETE statements.

The NOT EXISTS operator in SQL is the polar opposite of the EXISTS operator, and it is fulfilled if the subquery returns no results.

If the underlying subquery produces no results, the NOT EXISTS operation returns true. If the inner subquery matches a single record, the NOT EXISTS operator returns false, and the subquery execution can be terminated.

The NOT EXISTS condition in SQL Server is made up of two logical operators: EXISTS, which was previously discussed, and NOT, which negates a Boolean input.

SQL EXISTS syntax

SELECT
  column_name
FROM Table_Name
WHERE EXISTS (SELECT
  column_name
 FROM Table_Name
 WHERE condition);

SQL NOT EXISTS syntax

SELECT
  column_name
FROM Table_Name
WHERE NOT EXISTS (SELECT
  column_name
  FROM Table_Name
  WHERE condition);

Example 1: SQL EXISTS

Because the student_ id field of the student grade table matches the id column of the outer student table, the inner subquery is associated.

We can use the EXISTS SQL operator to filter student records with a 10 grade in Math, for example:

SELECT
  id, first_name, last_name
FROM
  student
WHERE EXISTS (
  SELECT 1
  FROM
   student_grade
  WHERE
   student_grade.student_id = student.id AND
   student_grade.grade = 10 AND
   student_grade.class_name = 'Math'
)
ORDER BY id

When running the query above, we can see that only the Alice row is selected:

SQL NOT EXISTS

Let's say we wish to choose all students with a grade of 9 or higher. We may do this by using NOT EXISTS, which contradicts the logic of the EXISTS operator.

To match all student records that have no associated student_grade with a value lower than 9, we can run the following SQL query:

SELECT
  id, first_name, last_name
FROM
  student
WHERE NOT EXISTS (
  SELECT 1
  FROM
   student_grade
  WHERE
   student_grade.student_id = student.id AND
   student_grade.grade < 9
)
ORDER BY id

When running the query above, we can see that only the Alice record is matched:

Example 2: SQL EXISTS example

Let’s consider the following example of SQL EXISTS usage. Suppose, our BicycleStoreDev database contains two tables: Customer and Order, that are linked with a one-to-many table relationship.

SELECT  *
FROM Sales.Customer c
WHERE EXISTS (SELECT *
FROM Sales.[Order] o
WHERE o.CustomerId = o.OrderId)

The query lists the rows from the Customer table in which CustomerID fields equal the OrderID fields in the Order table.

SQL NOT EXISTS example

We'll query the Customer table to locate entries where the CustomerID doesn't exist in the Order table to show how NOT EXISTS works in SQL Server. We can retrieve a list of customers who haven't placed any orders yet by using NOT EXISTS.

SELECT * 
FROM Sales.Customer c
WHERE NOT EXISTS (SELECT 
CustomerId
FROM Sales.[Order] o)

When testing for the existence of a result from a connected subquery, use the exists keyword.

It's not possible for the outer query to also be a subquery (For example, a nested subquery).

The equality operator, "=", must be used in the correlation criterion in the inner query. In the subquery, the equality operator must only return one row.

A group by or connect by reference cannot be used in the subquery.

The rule for changing an associated subquery automatically is straightforward. The only rule is that for each correlation condition, only one record is returned from the subquery.

However, just because Oracle does not change the linked subquery does not mean you can't manually transform it. This transformation usually entails shifting the subquery to the from clause of the surrounding query, so turning it into an in-line view. Additionally, rather than being relocated to the from clause, some forms of correlated subqueries can be immediately merged into the surrounding subquery.

Syntax:

The form of the WHERE clause that is linked to the subquery is:

{where | having} [not] exists (subquery) 

The outer query's where clause checks for the existence of the subquery's rows. If this is the case, the subquery does not return any results. The subquery does not provide any data, but instead returns a TRUE or FALSE response.

The subquery should only return one column.

Example 1: For example, the following query returns all employees with bad credit:

select
 ename
from
 emp e
where EXISTS
 (select
  null
 from
 bad_credit b
where
 e.empno=b.empno
   );

Example 2: correlated subquery as a loop (although that is not necessarily how it actually runs). Consider this query:

select e.*
from emp e
where Exists (select 1
  from dept d
  where e.eid = d.deptid
             );

It is saying: "For each emp record in the outer query, check if eid has a matching dept.deptid." If there is no match -- because e.eid is NULL or the value is not in dept, then it returns no rows.

Example 3: Retrieve a list of Acme employees who have submitted resumes to personnel for a different job placement.

SELECT   EID, ENAME
 FROM     EMP
 WHERE    EXISTS
 (SELECT RNAME
 FROM   RESUME
 WHERE  EMP.ENAME = RESUME.RNAME) ;

Output:

In this query, the subquery cannot be evaluated completely before the outer query is evaluated. Instead, we have a correlated subquery. For each row in EMPLOYEE, a join of EMPLOYEE and RESUME tables is performed (even though RESUME is the only table that appears in the subquery's FROM clause) to determine if there is a resume name in RESUME that matches a name in EMPLOYEE.

For example, for the first row in the EMPLOYEE table (ENAME = 'Smith, Mary') the subquery evaluates as "true" if at least one row in the RESUME table has RNAME = 'Steve, Mary'; otherwise, the expression evaluates as "false". Since there is a row in RESUME with RNAME = 'Steve, Mary', the expression is true and Roberta Smith's row is displayed. Each row in EMPLOYEE is evaluated in a similar manner.

Example 4: For example, this query finds the names of all the publishers who publish business books:

select pub_name 
from publishers 
where exists 
 (select * 
 from titles 
 where pub_id = publishers.pub_id 
 and type = "business")

Output:

Example 5: The following example finds all customers who have placed more than two orders:

SELECT
  customer_id,
  first_name,
  last_name
FROM
  sales.customers c
WHERE
  EXISTS (
  SELECT
  COUNT (*)
  FROM
   sales.orders o
  WHERE
   customer_id = c.customer_id
  GROUP BY
   customer_id
  HAVING
   COUNT (*) > 2
    )
ORDER BY
  first_name,
  last_name;

We had a correlated subquery in this case that returned consumers who placed more than two orders.

The subquery produces an empty result set if the number of orders placed by the client is less than or equal to two, causing the EXISTS operator to evaluate to FALSE.

The customer will be included in the result primarily based on the EXISTS operator's result.

A SQL query with a 'having' clause in the subquery of a 'exists' clause. That's the strange thing. There is no error, and the subquery can be used independently. The entire query, however, yields the identical results with and without the 'having' clause.

Example 1: In your qualifier, reverse the order of the aggregate and the constant (as hinted to by the error message).

SELECT [DISTINCT] AO,AT,AV FROM P3AT AS OAT
WHERE
 EXISTS(
 SELECT AO FROM P3AT
 WHERE
  AO = OAT.AO
 GROUP BY AO
 HAVING 8 = COUNT(*)
 );

To show that the DISTINCT is optional, I included it in square brackets.

The statement selects all rows from sets with eight distinct AOs. All such instances will be returned without the DISTINCT; with the DISTINCT, only rows with various combinations of AO, AT, and AV will be presented. You have no way of knowing which precise row has been given from a set with identical AO, AT, and AV combinations if you use the DISTINCT.

As an aside, is it worth changing the error message to something like:

Example 2: This is kind of what my query looks like:

SELECT X
 FROM A
WHERE exists (
  SELECT X, count(distinct Y)
    FROM B
  GROUP BY X
 HAVING count(distinct Y) > 2)  

So I'm trying to select the rows from A where X has more then two occurances of Y in B.

Example 3: If I group by in the exists query, it either returns nothing or returns too many results. Here's an example of how to use northwind. There are 4 rows out of a total of 6 when you execute the exist query. None are returned when using the exists command. Some queries, like this one, choose the exists select without the aggregate, so I receive all six results instead of the four I wanted.

SELECT Orders.ShipRegion, Orders.CustomerID
FROM Orders
where exists(
SELECT Orders2.OrderID
FROM Orders Orders2
INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID
WHERE Region = 'SP'
AND Orders.OrderID = Orders2.OrderID
group by Orders2.OrderID
having count(Orders2.OrderID) > 6
)

EXISTS returns true if the subquery returns NULL.

The Exists keyword compares all values in the associated sub query column, whereas the IN keyword evaluates true or false. Select 1 is another command that can be used with Exists.

Example 1: See the following example:

SELECT first_name,
	last_name
FROM
	customer
WHERE
	EXISTS( SELECT NULL )
ORDER BY
	first_name,
	last_name;

In this example, the subquery returned NULL, therefore, the query returned all rows from the customer table.

Example 2: The following example returns all rows from the customers table:

SELECT customer_id,
    first_name,
    last_name
FROM
    sales.customers
WHERE
    EXISTS (SELECT NULL)
ORDER BY
    first_name,
    last_name;

The subquery returned a result set with NULL, causing the EXISTS operator to evaluate to TRUE in this case. As a result, the entire query returns all customers table rows.

The difference between EXISTS with SELECT * and SELECT 1 is insignificant. In all instances, SQL Server generates comparable execution plans.

Same as (NOT) EXISTS (SELECT 1 ...) and (NOT) EXISTS (SELECT * ...).

If the subquery produces one or more records, EXISTS returns true. Even if NULL or 1/0 is returned. Only SELECT statements within the subquery operate with EXISTS.

SELECT * is a SELECT operator that does not start with a list of expressions or column names. Because the value of the subquery column does not matter, you can replace SELECT * with SELECT 1 to optimise performance (only the returned rows are important).

Note: Because the subquery is restarted for EVERY line in the external query table, SQL queries that use the EXISTS condition in PostgreSQL are exceedingly inefficient.

Example 1: Let’s check how the execution plans look like when using SELECT * and SELECT 1 in the subquery with the EXISTS operator.

SELECT *
FROM dbo.Users u
WHERE u.Location = 'Antartica'
AND EXISTS(SELECT 1 FROM dbo.Comments c WHERE u.Id = c.UserId)

Example 2: Adding TOP (1) to the select clause with the EXISTS operator may appear to minimise the amount of data necessary to satisfy the condition. If only one record is returned, it makes sense to validate and continue, resulting in a faster query time. This is not the case, however.

SELECT *
FROM [dbo].SomeTable AS a
WHERE EXISTS
(
    SELECT TOP (1) 1
    FROM [dbo].ExistsTable AS b
    WHERE a.contract_id = b.contract_id
);
DBCC DROPCLEANBUFFERS;

Example 3: EXAMPLE EXISTS CONDITION WITH SELECT OPERATOR

Let us consider a simple example. Below is the SELECT operator which uses PostgreSQL condition EXISTS:

SELECT *
FROM products
WHERE EXISTS (SELECT 1
FROM inventory
WHERE products.product_id = inventory.product_id);

The EXISTS condition in this PostgreSQL example returns all entries from the products database if the inventory table contains at least one entry with the matching product id. Because the resulting collection of columns has nothing to do with the EXISTS criterion, we used SELECT 1 in the subquery to increase efficiency (only the returned row counts).

Example 4: You may also write (NOT) EXISTS (SELECT 1/0...) and get the same result - no (division by zero) error, proving that the statement isn't even evaluated.

SELECT a, b, c
  FROM a_table
 WHERE EXISTS
  (SELECT 1   --- This nice '1' is what I have seen other people use
   FROM another_table
  WHERE another_table.b = a_table.b
)

Main Article :- Sql difference between EXISTS and IN

The IN operator compares the columns supplied before the IN keyword to the result of a subquery. The EXISTS operator, on the other hand, does not check for a match because it merely confirms the existence of data in a subquery.

Syntax for IN

SELECT column_names FROM table_name WHERE column_name IN (subquery);

Syntax for Exists

SELECT column_namesFROM table_nameWHERE [NOT] EXISTS (subquery);

The IN operator and SQL's EXISTS operators were compared. Both operators do the same task, but their internal operations differ. They work in different ways logically. Any of these can be chosen based on the requirements. However, if the data set is huge, the EXISTS operator is always suggested.

Using EXISTS clause in the CASE statement to check the existence of a record.

Example 1: CASE EXISTS :

SELECT * FROM dbo.CompanyMaster
WHERE AreaId=
(CASE WHEN EXISTS (SELECT BusinessId
  FROM dbo.AreaSubscription
  WHERE AreaSubscription.BusinessId = CompanyMaster.BusinessId) 
   THEN @AreaId ELSE AreaId END)
AND AreaId IN (SELECT [@Areas].AreaId FROM @Areas)

Example 2: The Case-When-Exists expression in Oracle is really handy. Here's an example of how to use it in a sub-select to return a status. This SQL checks for a match between the PS_PERSON and PSOPRDEFN records to determine the person status. The idea is that if the operator is not in PS_PERSON then they are not a true person in PeopleSoft.

select O.OPRID,
  O.EMPLID,
  case when exists (
   select 1
   from PS_PERSON P
   where P.EMPLID = O.EMPLID
    ) then 'Person' else 'Not a Person' end as PERSON_STATUS
from PSOPRDEFN O;

Example 3: Using EXISTS clause in the CASE statement to check the existence of a record:

DECLARE @CustId INT = 2
SELECT (CASE WHEN EXISTS(SELECT 1 FROM dbo.Customer WITH(NOLOCK)
WHERE CustId = @CustId) THEN 'Record Exists'
ELSE 'Record doesn''t Exists' END) AS [Employee?]

INSERT IF NOT EXISTS queries can be written in a single atomic statement, negating the requirement for a transaction and avoiding guidelines violations.

I want to insert a row in a single line, without needing a transaction, and only if the url isn't already in the table. I don't want any mistakes or warnings. I just want to run a standard INSERT statement and know that the url has been inserted into the table.

insert if not exists into url(url) values(...)

Example 1: if not exists insert sql

IF NOT EXISTS (SELECT * FROM EmailsRecebidos 
 WHERE De = @_DE
 AND Assunto = @_ASSUNTO
 AND Data = @_DATA)
BEGIN
 INSERT INTO EmailsRecebidos (De, Assunto, Data)
 VALUES (@_DE, @_ASSUNTO, @_DATA)
END

Example 2: I want to insert a new record into my table if does not exist. When I write this code for example:

insert into tablename (code) values ('1448523')
WHERE not exists(select * from tablename where code='1448523')

I get an error

Main Article :- Sql difference between NOT EXISTS and NOT IN

NOT IN and NOT EXISTS are NOT the same.

NOT IN and NOT EXISTS are two methods for filtering out rows that are existing in one table but not in another, and both can be used interchangeably. The only difference between the two is efficiency (which is determined by whether the larger table is the "outer" or "inner" in the query), but the results are identical.

A NULL cannot be compared to any other value in Oracle, including another NULL. As a result, if the result set being probed returns a NULL, a NOT IN operation will fail. In this situation, a NOT IN query returns 0 rows, whereas a NOT EXISTS query returns the rows present in the one table but not in the other table.

Example 1: NOT IN query

select r.owner, r.object_name from results_table r
 where r.owner not in (select q.owner from query_table q)
 order by 1,2;

NOT EXISTS query

select r.owner, r.object_name from results_table r
where not exists (select '1' from  query_table q where r.owner=q.owner)
order by 1,2;

Example 2: NOT IN

select count(*) from emp_master where emp_nbr not in ( select mgr_nbr from emp_master );

Output:

This means that everyone is a manager…hmmm, I wonder whether anything ever gets done in that case

NOT EXISTS

select count(*) from emp_master T1 where not exists ( select 1 from emp_master T2 where t2.mgr_nbr = t1.emp_nbr );

Output:

There are currently nine people who are not managers. As you can see, NULL values make a big difference, and because NULL!= NULL in SQL, the NOT IN clause doesn't return any records. (The behaviour of MS SQL Server can vary based on the ANSI NULLS setting, however this post only covers the behaviour that is consistent throughout Oracle, DB2 LUW, and MS SQL Server).

Example 3:

select count(*) from emp where empno not in ( select mgr from emp );

Output:

apparently there are NO rows such that an employee is not a mgr -- everyone is a mgr (or are they)

select count(*) from emp T1
where not exists ( select null from emp T2 where t2.mgr = t1.empno );

Output:

However, there are currently nine people who are not managers. Be cautious of NULL and NOT IN values (also the reason why NOT IN is sometimes avoided). If an "anti-join" can be utilised, NOT IN can be just as efficient as NOT EXISTS —— if not better (if the subquery is known to not return nulls).