Query stored procedure mysql

Preface

When developing database applications, most complex operations of database queries can be implemented using stored procedures. A query stored procedure is a reusable block of code that is a predefined SQL statement and is stored in the database. Developers can handle complex query operations by calling stored procedures. MySQL is a very popular open source database management system that also supports the use of stored procedures.

This article will briefly introduce the use of query stored procedures in MySQL.

1. Create a stored procedure

To create a stored procedure using MySQL, you need to use the CREATE PROCEDURE statement. The syntax is as follows:

CREATE [DEFINER = { user | CURRENT_USER }] PROCEDURE procedure_name ([proc_parameter[,...]])
BEGIN
   /* 存储过程主体 */
END;

Among them, DEFINER defines the owner of the stored procedure, i.e. the user who created it. procedure_name is the name of the stored procedure. proc_parameter is an optional stored procedure parameter that can be passed to the stored procedure.

The following is an example of a simple query stored procedure:

CREATE PROCEDURE sp_GetUserById(IN userId INT)
BEGIN
   SELECT * FROM Users WHERE Id = userId;
END;

The name of this stored procedure is sp_GetUserById. It receives an integer parameter userId and queries the user table based on the passed in parameter value. Record.

2. Execute the stored procedure

To execute the stored procedure, you can use the CALL statement, the syntax is as follows:

CALL procedure_name([parameter[,...]]);

Among them, procedure_name is the name of the stored procedure to be executed, and the parameters Can be any parameter type defined by the stored procedure.

Using the query stored procedure example above, you can execute the following call:

CALL sp_GetUserById(1);

This will return the record with Id=1 in the user table.

3. Delete stored procedures

To delete a stored procedure, you can use the DROP PROCEDURE statement. The syntax is as follows:

DROP PROCEDURE [IF EXISTS] procedure_name;

Among them, procedure_name is the name of the stored procedure to be deleted. The IF EXISTS option avoids error messages when the stored procedure does not exist.

For example, to delete the stored procedure created earlier, you can use the following command:

DROP PROCEDURE IF EXISTS sp_GetUserById;

This will delete the stored procedure named sp_GetUserById (if it exists).

4. Using a cursor

When executing a query in a stored procedure, sometimes you need to use a cursor to iterate the result set. Cursor usage in MySQL is similar to standard SQL.

The following is an example of using a cursor:

CREATE PROCEDURE sp_GetAllUsers()
BEGIN
    /* 声明游标、变量 */
    DECLARE curUsers CURSOR FOR SELECT * FROM Users;
    DECLARE done INT DEFAULT FALSE;
    DECLARE userId INT;
    DECLARE userName VARCHAR(50);

    /* 打开游标 */
    OPEN curUsers;

    /* 迭代结果集 */
    REPEAT
        /* 获取下一行 */
        FETCH curUsers INTO userId, userName;

        /* 如果结果集为空，则退出循环 */
        IF done THEN
            LEAVE main_loop;
        END IF;

        /* 处理当前行 */
        SELECT userId, userName;

    /* 主循环，退出标签 */
    UNTIL done END REPEAT;

    /* 关闭游标 */
    CLOSE curUsers;

    /* 结束存储过程 */
END;

This stored procedure will query all records in the user table and use the cursor to iterate the result set. After obtaining each row of data, userId and userName will be output.

5. Use process control

Various process control structures can be used in stored procedures, such as IF-THEN, WHILE and REPEAT-UNTIL.

The following is an example of using the IF-THEN structure:

CREATE PROCEDURE sp_GetUsersByRole(IN roleId INT)
BEGIN
   IF roleId IS NULL THEN
      SELECT * FROM Users;
   ELSE
      SELECT * FROM Users WHERE RoleId = roleId;
   END IF;
END;

This stored procedure will query the records in the user table based on the passed parameter value roleId. If roleId is NULL, all records will be returned.

Conclusion

Query stored procedures are one of the very useful features of the MySQL database, which can be used to perform complex query operations. Each stored procedure can be used as an independent block of code and can be reused in the code.

This article introduces the creation, execution and deletion of stored procedures, the use of cursors and process control structures, and a simple example. However, it should be noted that the design of stored procedures should follow best practices, including proper checking and processing of input and output parameters, as well as ensuring security and performance issues.

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