(16) The variables in MySQL

Variable

System variables and custom variables

System variables

System variables: System-defined variables, most of the system variables, generally do not require user control, system variables used to control the performance of the server such as: Autocommit, auto_increment and so on.

viewing system Variables

Show variables;

View variable values for a specific variable: any content that has data returned is a select view

View basic syntax for system variables: select @@ 变量 name;

Eg:select @ @version, @ @autocommit, @ @auto_increment_offset, @ @character_set_results;

modifying system Variables

There are two ways to modify system variables: Session level and global level

Set @@ 变量 name = value;

Set variable name = value;

Session level: Temporary modification, the current client is valid, when the secondary connection is valid.

Global level: One modification, permanent (valid for all clients)

Set global variable name = value;

Other clients need to restart the client to take effect.

Custom variables

In order to differentiate system variables, the system specifies that the custom variable must use an @ symbol.

SET @ variable name = value;

Select variable name;

In MySQL, "=" would implicitly consider the comparison of symbolic processing, and MySQL redefined a new assignment symbol for the concept of distinguishing between comparison and assignment---- : =

MYSQL allows you to get data from a data table and assign it to a variable : two ways

Scenario 1: Edge assignment, side view results

SELECT @ Variable Name: = Field name form data source; --Assigns a field from the database table to the variable.

Scenario 2: Only the assignment does not look at the result, it is strict: only one data record can be obtained, MySQL does not support data.

Select field List from table name into variable list;

The custom variables used are session-level. Variables are not database-sensitive and belong to the user level.

(16) The variables in MySQL