How to declare a variable in MySQL for a normal query?

You can declare a variable using @anyVariablename which is a session variable. To create a session variable, you need to use SET command.

The syntax is as follows

SET @anyVariableName:=anyValue;

You can declare a local variable using DECLARE command. The syntax is as follows

DECLARE yourVariableName datatype

You can set the default value at the time of creating a variable. The syntax is as follows

DECLARE yourVariableName datatype default ‘yourValue’

Here is the demo of session variable. To understand it, let us create a table.

The query to create a table is as follows

mysql> create table SessionVariableDemo
   -> (
   -> EmployeeId varchar(10),
   -> EmployeeName varchar(30),
   -> EmployeeAge int
   -> );
Query OK, 0 rows affected (0.70 sec)

Insert some records in the table using insert command. The query is as follows

mysql> insert into SessionVariableDemo values('EMP-101','Carol',30);
Query OK, 1 row affected (0.20 sec)

mysql> insert into SessionVariableDemo values('EMP-102','John',26);
Query OK, 1 row affected (0.20 sec)

mysql> insert into SessionVariableDemo values('EMP-103','Bob',25);
Query OK, 1 row affected (0.19 sec)

mysql> insert into SessionVariableDemo values('EMP-104','Sam',32);
Query OK, 1 row affected (0.14 sec)

mysql> insert into SessionVariableDemo values('EMP-105','Mike',35);
Query OK, 1 row affected (0.11 sec)

mysql> insert into SessionVariableDemo values('EMP-106','David',33);
Query OK, 1 row affected (0.15 sec)

Display all records from the table using select statement. The query is as follows

mysql> select *from SessionVariableDemo;

The following is the output

+------------+--------------+-------------+
| EmployeeId | EmployeeName | EmployeeAge |
+------------+--------------+-------------+
| EMP-101    | Carol        |          30 |
| EMP-102    | John         |          26 |
| EMP-103    | Bob          |          25 |
| EMP-104    | Sam          |          32 |
| EMP-105    | Mike         |          35 |
| EMP-106    | David        |          33 |
+------------+--------------+-------------+
6 rows in set (0.00 sec)

Now, create a session variable using SET command. After that we will use this variable in the query to get all employee records with age greater than 30.

Let us create a session variable using SET command

mysql> set @AgeGreaterThan30:=30;
Query OK, 0 rows affected (0.00 sec)

Here is the query that will use the session variable to get the employee records with age greater than 30

mysql> select *from SessionVariableDemo where EmployeeAge > @AgeGreaterThan30;

The following is the output

+------------+--------------+-------------+
| EmployeeId | EmployeeName | EmployeeAge |
+------------+--------------+-------------+
| EMP-104    | Sam          |          32 |
| EMP-105    | Mike         |          35 |
| EMP-106    | David        |          33 |
+------------+--------------+-------------+
3 rows in set (0.00 sec)