Detailed introduction to the return value issue of MySQL stored procedures

In using MySQL for data development, stored procedures are a very commonly used tool. Stored procedures can reduce code redundancy, simplify operating procedures, and improve code operating efficiency, thereby increasing the efficiency of data development. When using stored procedures, we often need to consider the return value of the stored procedure. This article will introduce in detail the return value issue of MySQL stored procedures.

1. The basic structure of MySQL stored procedures

In MySQL, a stored procedure consists of the following parts:

1. Stored procedure name: that is, the stored procedure The unique identifier of the procedure in MySQL.
2. Stored procedure parameter list: used to receive external incoming parameters to complete specific operations.
3. Stored procedure body: Indicates the specific execution logic of the stored procedure, which can include SQL statements, conditional judgments, loop structures, etc.
4. Stored procedure return value: Indicates the value returned to the caller after the execution of the stored procedure is completed.

2. Return value types of MySQL stored procedures

In MySQL, the return value types of stored procedures can be divided into the following three types:

1. Fixed number of return values ​​(OUT)

This type of return value can be understood as the only value returned by the stored procedure. When defining a stored procedure, we can add OUT type parameters to the parameter list to receive return values. For example:

DELIMITER $$ CREATE PROCEDURE GetTotal(IN a INT, IN b INT, OUT total INT) BEGIN SET total = a b; END$$ DELIMITER ;

Stored procedure GetTotal, receives two Integer parameters a and b, calculate the sum of the two parameters, and return the result to the caller.

Note: When defining a stored procedure, DELIMITER should be defined as other characters (characters other than semicolon) to avoid semicolons in SQL statements being misunderstood as delimiters.

2. Multiple return values ​​(INOUT)

This type of return value can be understood as a stored procedure returning multiple values ​​(including both input parameters and output parameters). When defining a stored procedure, we can add INOUT type parameters to the parameter list. For example:

DELIMITER $$ CREATE PROCEDURE CompareAge(INOUT age1 INT, INOUT age2 INT) BEGIN IF age1 < age2 THEN SET age1 = age2; SET age2 = age1; END IF; END$$ DELIMITER ;

The stored procedure CompareAge receives two integer parameters age1 and age2, compares the sizes of the two parameters, and if age1 is less than age2, exchanges the values ​​of the two parameters.

When executing a stored procedure, we need to set initial values ​​for parameters first, and then pass the parameters to the stored procedure. After the stored procedure is executed, the results can be updated to the original parameters.

3. Return result set (CURSOR)

This type of return value can be understood as returning a result set. When defining a stored procedure, you need to define a cursor, and use the cursor to return the result set. For example:

DELIMITER $$ CREATE PROCEDURE GetList() BEGIN DECLARE cur CURSOR FOR SELECT * FROM TABLE_NAME; OPEN cur; SELECT * FROM TABLE_NAME; CLOSE cur; END$$ DELIMITER ;

Stored Procedure GetList, queries the records of the TABLE_NAME table and returns the result set to the caller.

Note: When using the result set, you should use the CALL statement to call the stored procedure, such as: CALL GetList().

3. Execution of MySQL stored procedures

The execution of stored procedures is divided into two phases: the compilation phase of the stored procedure and the running phase of the stored procedure.

1. The compilation phase of stored procedures

The compilation phase of stored procedures refers to the fact that MySQL will compile the stored procedure when it retrieves it for the first time. During the compilation process, MySQL performs syntax analysis and semantic analysis on the stored procedure and generates an execution plan.

2. The running phase of the stored procedure

The running phase of the stored procedure refers to when the stored procedure is called, MySQL will execute the generated execution plan and return the execution results to caller. During the execution process, MySQL will first execute the statement of the stored procedure body, and then return the corresponding result according to the return value type of the stored procedure.

4. Use of MySQL stored procedure return values

When using the return value of a stored procedure, we need to use the CALL statement to call the stored procedure and pass the parameters to the stored procedure. For example:

CALL GetTotal(1, 2, @result); SELECT @result;

After executing the stored procedure, we can get the return value of the stored procedure through the @variable name. @result here is the return value of the stored procedure GetTotal.

Note: When using a stored procedure to return a result set, you should use the CALL statement to call the stored procedure and use a cursor to process the result set.

5. Summary

This article introduces the return value issue of MySQL stored procedure in detail, including the basic structure of the stored procedure, return value type, execution process and use of return value. When using stored procedures, we should choose the appropriate return value type according to actual needs, and use the CALL statement to call the stored procedure, so as to achieve high efficiency in data development.

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