SQL UNION

-- select the union of name columns from two tables Teachers and Students 
SELECT name
FROM Teachers
UNION
SELECT name
FROM Students;

Here, the SQL command selects the union of the name columns from two different tables: Teachers and Students.

SQL Union Syntax

The syntax of the SQL UNION command is:

SELECT column1, column2, ...
FROM table1
UNION
SELECT column1, column2, ...
FROM table2;

Here,

• column1,column2, ... are the column names required for the union
• table1 and table2 are the names of the tables to fetch the columns from
• UNION combines the columns in the tables

Note: If the selected columns from the tables contain the same data, those columns are only included once in the result set.

Example: SQL UNION

-- select the union of age columns from two tables Teachers and Students 
SELECT age
FROM Teachers
UNION
SELECT age
FROM Students;

Here, the SQL command returns the age columns from the Teachers and the Students tables, ignoring the duplicate fields.

Things to Note While Using UNION

To use UNION in SQL, we must always remember,

• The column count in all tables must be the same. For example, both the Teachers and Students tables have three columns.
• The data types of columns must be the same. For example, the age column in Teachers is integer, so is the age in the Students table.

• The columns must be in the same order in each table. For example, the order of columns is id-name-age in both Teachers and Students tables.

Note: Our online compiler is based on SQLite, which converts the values in one of the columns to match the data type of the other column while performing a UNION operation.

SQL UNION ALL Operator

The UNION ALL operator selects fields from two or more tables similar to UNION. However, unlike UNION, UNION ALL doesn't ignore duplicate fields.

Let's try the previous SQL command again using UNION ALL instead of UNION.

-- select the union of age from Teachers and Students tables
 
SELECT age
FROM Teachers
UNION ALL
SELECT age
FROM Students;

Here, the SQL command selects fields from both tables, including the duplicate fields.

SQL UNION vs. UNION ALL

For improved performance, It is recommended you use UNION ALL when you know the selected fields will only have unique values.

SQL UNION vs. SQL JOIN

To learn more, visit SQL JOIN.