mysql stored procedure cursor

Detailed explanation of MySQL stored procedures and cursors

In the MySQL database, a stored procedure is a set of pre-compiled SQL statements, which can be regarded as a subroutine in the database. Stored procedures are usually used to handle complex business logic, simplifying the interaction between applications and databases, and improving data processing efficiency and security. A cursor is a mechanism used to process query result sets in stored procedures.

This article will introduce in detail the usage and characteristics of MySQL stored procedures and cursors.

1. Stored procedures

1.1 Syntax structure of stored procedures

The following is the basic syntax structure of MySQL stored procedures:

CREATE PROCEDURE procedure_name [ (parameter_list) ] BEGIN
    -- 存储过程的逻辑实现
END;

where ,

• procedure_name: the name of the stored procedure;
• parameter_list: the parameter list of the stored procedure, which can contain 0 or more parameters. Each parameter consists of a parameter name and a parameter type. Composition;
• Between BEGIN and END is the logical implementation part of the stored procedure.

For example, define a simple stored procedure to query all records in the student table (student) and return:

CREATE PROCEDURE get_all_students()
BEGIN
    SELECT * FROM student;
END;

1.2 Parameter passing of stored procedures

Stored procedures can predefine parameters, which can be passed when calling the stored procedure. MySQL stored procedures support three parameter passing methods: IN, OUT and INOUT.

• IN: As an input parameter, the parameter value is passed when calling the stored procedure.
• OUT: As an output parameter, the result is calculated and returned inside the stored procedure.
• INOUT: It is both an input parameter and an output parameter. When calling a stored procedure, the parameter value is passed and the result calculated by the stored procedure is received.

The following is an example of a stored procedure using parameter passing:

CREATE PROCEDURE add_two_numbers(IN num1 INT, IN num2 INT, OUT result INT)
BEGIN
    SET result = num1 + num2;
END;

In the above example, the add_two_numbers stored procedure receives two input parameters num1 and num2, and the result of adding them is Assign a value to the output parameter result.

You can use the following statements to call this stored procedure:

CALL add_two_numbers(2, 3, @result);
SELECT @result; -- 输出 5

1.3 Process control of stored procedures

MySQL stored procedures support common process control statements, such as IF, CASE, WHILE, LOOP and LEAVE etc. Use these statements to implement complex logic control and business processing.

The following is an example of a stored procedure using IF and WHILE statements:

CREATE PROCEDURE calc_factorial(IN num INT, OUT result BIGINT)
BEGIN
    SET result = 1;
    WHILE num > 1 DO
        SET result = result * num;
        SET num = num - 1;
    END WHILE;
END;

In the above example, if the input parameter num is greater than 1, the WHILE loop is used to calculate the factorial value of num, and Store the result in the output parameter result.

You can use the following statement to call this stored procedure:

CALL calc_factorial(6, @result);
SELECT @result; -- 输出 720

2. Cursor

2.1 The basic concept of cursor

In MySQL stored procedure , a cursor is the mechanism used to traverse a query result set in a stored procedure. The cursor tracks the current row in the query result set and moves row by row as needed to read the data in that row.

Using a cursor to declare, open, close and operate a cursor requires the following four commands:

• DECLARE: declare a cursor, define the name of the cursor, query statement and cursor type, etc. .
• OPEN: Open the cursor and save the query result set to the buffer pointed by the cursor.
• FETCH: Get the current row pointed by the cursor and store the value of the current row in the corresponding variable.
• CLOSE: Close the cursor and release the memory occupied by the cursor.

The following is an example of a stored procedure using a cursor:

CREATE PROCEDURE get_all_students()
BEGIN
    DECLARE done INT DEFAULT FALSE; -- 定义游标是否结束的标志
    DECLARE s_id INT; -- 存储查询结果中的学生编号
    DECLARE s_name VARCHAR(255); -- 存储查询结果中的学生姓名
    DECLARE cursor_students CURSOR FOR SELECT id, name FROM student; -- 声明游标，查询表 student 中的所有数据
    -- 打开游标
    OPEN cursor_students;
    -- 遍历游标指向的结果集
    read_loop: LOOP
        -- 获取游标指向的当前行
        FETCH cursor_students INTO s_id, s_name;
        -- 如果游标到达结果集的末尾，则跳出循环
        IF done THEN
            LEAVE read_loop;
        END IF;
        -- 输出当前行的值
        SELECT s_id, s_name;
    END LOOP;
    -- 关闭游标
    CLOSE cursor_students;
END;

In the above example, a cursor named cursor_students is declared to query all data in the table student. After opening the cursor, use the read_loop loop body to traverse the query result set, and use the FETCH command to obtain the value of the current row, store it in the variables s_id and s_name, and output it to the console. If the cursor reaches the end of the result set, break out of the loop. When the cursor is finished using, you need to use the CLOSE command to close the cursor.

2.2 Characteristics and application scenarios of cursors

Although cursors provide developers with a convenient way to query result sets, because they require additional memory and resources, they must be used in You need to pay special attention to the following issues:

• Cursors will affect performance, especially when processing large data sets. It is not recommended to use cursors in large-scale or high-concurrency environments, and other methods, such as subqueries and JOIN operations, should be given priority.
• Cursors can only be used in stored procedures and cannot be used directly in SQL statements.
• You need to be careful when using cursors, because if the cursor is not closed correctly, it will cause the MySQL database to occupy a lot of memory resources and even crash.

Normally, cursors are suitable for the following scenarios:

• Situations where complex query logic needs to be implemented in stored procedures.
• Situations where large data sets need to be processed in batches.
• Situations where the query result set needs to be processed row by row.

3. Summary

This article mainly introduces the usage and characteristics of stored procedures and cursors in MySQL database. Stored procedures can improve the efficiency and security of interactions between applications and databases, and cursors can easily traverse query result sets. However, it should be noted that you need to be careful when using cursors to avoid memory leaks and performance problems.

References:

• MySQL :: MySQL 8.0 Reference Manual :: 13.6.4.1 DECLARE cursor Statement
• MySQL :: MySQL 8.0 Reference Manual :: 13.6.4.2 OPEN cursor Statement
• MySQL :: MySQL 8.0 Reference Manual :: 13.6.4.3 FETCH cursor Statement
• MySQL :: MySQL 8.0 Reference Manual :: 13.6.4.4 CLOSE cursor Statement

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