mysql returns stored procedure

MySQL is an open source and powerful relational database management system. It supports stored procedures, which are precompiled blocks of SQL code that perform some specific function or task in the database. Stored procedures can reduce duplicate code, improve the efficiency of database operations, and ensure security.

This article will focus on the use of stored procedures and the processing of return values in MySQL.

1. Creation and use of MySQL stored procedures
Creating a MySQL stored procedure includes the following steps:

1. Define the stored procedure name and parameters
   The specific implementation method is:

CREATE PROCEDURE procedure_name (IN parameter_name DATA_TYPE)

Among them, PROCEDURE is the keyword to create the stored procedure, procedure_name is the name of the stored procedure, IN parameter_name is the name of the input parameter, and DATA_TYPE is the data type of the input parameter.

2. Specify the stored procedure statement
   The specific implementation method is:

BEGIN -- 存储过程语句 END

Among them, BEGIN and END are the keywords for specifying the stored procedure statement. Multiple SQL statements can be written between BEGIN and END to implement the specific functions of the stored procedure.

3. Return results
   In a stored procedure, you can use the SELECT statement to return results. As shown below:

CREATE PROCEDURE procedure_name (IN parameter_name DATA_TYPE) BEGIN SELECT * FROM table_name WHERE column_name = parameter_name; END

Among them, * is a wildcard character, which means returning data of all columns; table_name is the table name; column_name is the column name.

2. Calling MySQL stored procedures
After creating a MySQL stored procedure, you can use the CALL statement to call the stored procedure. The specific implementation method is:

CALL procedure_name(parameter_value);

Among them, CALL is the keyword for calling the stored procedure; procedure_name is the name of the stored procedure; parameter_value is the parameter value passed to the stored procedure.

3. Return value processing of MySQL stored procedures
MySQL stored procedures can return two types of values: single value and result set. For a single value, you can use the OUT parameter to return its value, and for a result set, you can use CURSOR.

1. Return a single value through the OUT parameter
   The specific implementation method is to declare the OUT parameter in the CREATE PROCEDURE statement, as shown below:

CREATE PROCEDURE procedure_name (IN parameter_name DATA_TYPE, OUT out_parameter_name DATA_TYPE)

Among them, OUT is the statement Keyword of OUT parameter; out_parameter_name is the name of the output parameter, and DATA_TYPE is the data type of the output parameter.

In the stored procedure statement, you can use the SET statement to set the value of the OUT parameter, as shown below:

CREATE PROCEDURE procedure_name (IN parameter_name DATA_TYPE, OUT out_parameter_name DATA_TYPE) BEGIN SET out_parameter_name = (SELECT COUNT(*) FROM table_name WHERE column_name = parameter_name); END

Among them, COUNT(*) is an aggregate function, returning the total number of records that meet the conditions .

When calling a stored procedure, you can use the SELECT statement to retrieve the value of the OUT parameter, as shown below:

CALL procedure_name(parameter_value, @out_parameter_value); SELECT @out_parameter_value;

Among them, @out_parameter_value is a user variable used to store the value of the OUT parameter.

2. Return the result set through CURSOR
   CURSOR is a data type used to traverse the result set. In MySQL stored procedures, you can use the DECLARE, OPEN, FETCH, CLOSE and END keywords to define CURSOR and result sets.

The specific implementation method is:

CREATE PROCEDURE procedure_name () BEGIN DECLARE cursor_name CURSOR FOR SELECT column_name FROM table_name; OPEN cursor_name; FETCH cursor_name INTO variable_name; WHILE @@FETCH_STATUS = 0 DO -- 处理结果集 FETCH cursor_name INTO variable_name; END WHILE; CLOSE cursor_name; END

Among them, DECLARE is the keyword to declare CURSOR; FOR SELECT column_name FROM table_name is the result set query statement; OPEN is the keyword to open CURSOR; FETCH is the keyword to obtain the next row of data in CURSOR; WHILE @@FETCH_STATUS = 0 is the condition to check whether the entire result set has been traversed; CLOSE is the keyword to close CURSOR.

When calling a stored procedure, you can use the SELECT statement to retrieve data in the CURSOR, as shown below:

CALL procedure_name(); SELECT column_name FROM table_name WHERE condition;

4. Precautions for MySQL stored procedures
In the usage process of MySQL stored procedures , you need to pay attention to the following points:

1. The variable name in the stored procedure should not have the same name as the keyword.
2. You can use the SET statement to set the value of a variable in a stored procedure.
3. Semicolons can be used to separate multiple statements in a stored procedure.
4. Do not have the same parameter names in the stored procedure and column names in the data table, otherwise ambiguity may occur.
5. Be careful to add parentheses () when using stored procedures, otherwise syntax errors may occur.

Summary:
MySQL stored procedure is a pre-compiled block of SQL code that can perform certain specific functions or tasks in the database. Stored procedures can reduce duplicate code, improve the efficiency of database operations, and ensure security. This article introduces the creation, calling and return value processing of MySQL stored procedures, as well as matters needing attention when using stored procedures. Through the flexible use of MySQL stored procedures, developers can help developers complete database operations more efficiently.

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