# Custom variables

In MatrixOne, custom variables are a mechanism for storing and manipulating values. Custom variables can be set through the `SET` statement and can be kept the value unchanged throughout the session. You can customize variables through the `SET` command and use them in SQL. The specific syntax is as follows:

```sql
SET @variable_name = value;
```

`@variable_name` is the name of the custom variable, and `value` is the value to be assigned to that variable. Once a variable is defined, it can be used in the SQL statement instead of the actual value.

For example, the following statement will define a variable named `@max_salary` and set it to 100000:

```text
SET @max_salary = 100000;
-- View the value set by @max_salary variable
mysql> select @max_salary;
+-----------------+
| @max_salary |
+-----------------+
| 100000 |
+-----------------+
1 row in set (0.01 sec)
```

When using custom variables, you can include them in SQL statements, referenced in the form of `@variable_name`. For example, the following statement will return all employee records with salary less than `@max_salary`:

```sql
SELECT * FROM employees WHERE salary < @max_salary;
```

The results of SQL queries can be affected by changing the value of a custom variable. For example, the following statement changes the value of `@max_salary` and returns a new query result:

```sql
SET @max_salary = 80000;
SELECT * FROM employees WHERE salary < @max_salary;
```

It should be noted that custom variables are only valid in the current session, and when the session ends, the variables will be deleted and released. Additionally, variable names must start with the `@` symbol and are case sensitive.

## Simple example

Now take the example of defining two variables a and b:

```text
> SET @a=2, @b=3;
Query OK, 0 rows affected (0.00 sec)

> select @a;
+------+
| @a |
+------+
| 2 |
+------+
1 row in set (0.00 sec)

> select @b;
+------+
| @b |
+------+
| 3 |
+------+
1 row in set (0.00 sec)
```

Use custom variables in SQL:

```text
> create table t1(a int,b varchar(1));
Query OK, 0 rows affected (0.02 sec)

> insert into t1 values(@a,@b);
Query OK, 1 row affected (0.02 sec)

> select * from t1;
+------+------+
| a | b |
+------+------+
| 2 | 3 |
+------+------+
1 row in set (0.01 sec)
```

:::{note}
The variables a and b are both int data types here. If you want 2 or 3 of a string, it is recommended to use `SET @a='2', @b='3';`.
:::
## MySQL Compatibility

MatrixOne supports session level, the same as MySQL support.