SQL JOIN Clause Vs UNION Operator

In a relational database management system, the SQL keywords UNION and JOIN are used to perform operations on many tables (RDBMS).

They combine data from two or more tables to get a result. However, in both phrases, the method for combining data from two or more relations differs. Before making a comparison, let's take a quick look at these clauses.


In MySQL, a JOIN is used in conjunction with the SELECT query to extract data from many tables.

It is used if we need to retrieve records from many tables. It only returns records from the tables that meet the provided criteria.

A JOIN is used to display columns from various tables that have the same or different names.

All of the columns will be demonstrated independently in the output. In other words, the columns will be lined next to one another.

Tables in a relational database are linked to one another, and we used foreign keys to keep track of the relationships between them. The join condition specifies how each table's columns are compared to one another.

Example 3: Assume our database comprises the following tables: "Student" and "Technologies," each of which has the following information:

Using the following query, we can get records from both tables:

SELECT Student.name, Technologie.technology      
FROM Student     
JOIN Technologie   
ON Student.id = Technologie.id;  


The MySQL UNION clause allows us to aggregate two or more relations into a single result set by performing multiple SELECT queries.

It features a feature that removes duplicate rows from the result set by default.

In MySQL, the union clause must adhere to the following guidelines:

  • In all tables, the arrangement and number of columns must be the same.
  • Each select query's data type must be consistent with the associated positions.
  • In SELECT queries, the column names should be in the same sequence.

The UNION set operator is used to join data from two tables with columns of the same datatype.

When you do a UNION , the data from both tables is combined into a single column with the same datatype.

Example: By joining both tables, the following statement generates output that contains all student names and subjects.

SELECT Name, subject FROM student1    
SELECT Name, subject FROM student2;   


There are 10 main differences between join clause & union operator:-

JOIN joins data from multiple tables based on a matching criteria. SQL aggregates the results of two or more SELECT operations into a single result set.
It creates new columns by combining data. It creates new rows by combining data.
The number of columns chosen from each table may differ. Each table's number of columns should be the same.
The datatypes of the corresponding columns in each table may differ. The datatypes of the corresponding columns in each table should be the same.
It's possible that it won't return distinct columns. Between the several select statements, it removes duplicate rows.
JOIN produces a new horizontal collection of rows with the same number of rows but varying numbers of columns. UNION produces a new vertical set of rows with the same number of columns but a variable number of rows.
When the tables involved have similar attributes for at least one field, JOIN joins the data. When two tables have the same number of attributes and the domains of corresponding attributes are also the same, the UNION in SQL is used.
Inner join, left join, right join, full join, and cross join are examples of JOIN variations. UNION and UNION ALL are two variations of UNION. INTERCEPT and MINUS/EXCEPT could be considered versions of UNION in that they integrate data vertically as well, similar to what UNION does.
In a join condition, primary keys and foreign keys are usually used. With the exception of point 3 above, UNION does not have a union condition.
It's a combination of many tables that adds up all of the records. The intersection of the tables yields results.