mysql stored procedure loop

The loop of MySQL stored procedures is a very important language structure that can be used in stored procedures so that the stored procedures can perform specific operations for different conditions. This article will introduce the loop structure of MySQL stored procedures, as well as its usage scenarios and usage, to help developers better understand and apply the MySQL stored procedure language.

1. The loop structure of MySQL stored procedures

There are two types of loop structures of MySQL stored procedures: WHILE loop and FOR loop. Below we introduce their grammatical structures and usage scenarios in turn.

1. WHILE loop

WHILE loop is the most basic loop structure, and its syntax is as follows:

WHILE condition DO
-- 循环体语句
END WHILE;

Where, condition is a logical expression, if If the expression is true, the loop body statement is executed. Each time the loop body statement is executed, the value of condition will be recalculated. Until condition is no longer true, the loop body will be jumped out and the statement after END WHILE will be executed.

For example, the following stored procedure uses the WHILE loop to calculate the sum of numbers from 1 to n:

CREATE PROCEDURE sum(n INT)
BEGIN
DECLARE i INT DEFAULT 1;
DECLARE total INT DEFAULT 0;
WHILE i <= n DO
SET total = total + i;
SET i = i + 1;
END WHILE;
SELECT total;
END;

Among them, the variables i and total are used to calculate the sum value and the number of loops respectively. When i <= n, execute the loop body statement:

SET total = total + i;
SET i = i + 1;

Every time this statement is executed, the values ​​of i and total will be recalculated. Until i > n, the loop body will be jumped out and the SELECT statement will be executed. , returns the sum value.

2. FOR loop

FOR loop is a more concise loop structure than WHILE loop. Its syntax is as follows:

FOR var_name [, var_name] ...  IN range DO
-- 循环体语句
END FOR;

Among them, var_name is the loop The name of the counter, range represents the range of the loop counter. range can be in the following format:

• start_value TO end_value: Starting from start_value and ending at end_value, the loop counter increases by 1 each time.
• start_value TO end_value BY step: Starting from start_value and ending at end_value, the loop counter increases step each time.

For example, the following stored procedure uses a FOR loop to calculate the square of a number from 1 to n:

CREATE PROCEDURE square(n INT)
BEGIN
DECLARE i INT;
DECLARE v INT;
FOR i IN 1 TO n DO
SET v = i * i;
SELECT v;
END FOR;
END;

Among them, the loop counter i goes from 1 to n, increasing by 1 each time. Execute the loop body statement:

SET v = i * i;
SELECT v;

Every time this statement is executed, the value of v will be recalculated. Until i > n, the loop body will be jumped out and the END statement will be executed.

2. Cyclic usage scenarios of MySQL stored procedures

The cyclic structure of MySQL stored procedures is more commonly used in the following scenarios:

1. Data batch processing

The loop structure of MySQL stored procedures can be used to process data in batches, process multiple pieces of data at one time, and improve the efficiency and performance of data processing.

For example, the following stored procedure uses a WHILE loop to reduce the prices of products in the product table with prices greater than 100 by 5 in batches:

CREATE PROCEDURE update_price()
BEGIN
DECLARE p_id INT;
DECLARE p_price DECIMAL(10,2);
DECLARE done INT DEFAULT FALSE;
DECLARE cur CURSOR FOR SELECT id, price FROM product WHERE price > 100;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
OPEN cur;
REPEAT
FETCH cur INTO p_id, p_price;
IF NOT done THEN
SET p_price = p_price - 5;
UPDATE product SET price = p_price WHERE id = p_id;
END IF;
UNTIL done END REPEAT;
CLOSE cur;
END;

A cursor cur is defined for traversing product Items with a price greater than 100 in the table. Each time through the loop, a piece of data is obtained from the cursor, and if p_price > 100, the product price is updated.

2. Data analysis and statistics

The loop structure of MySQL stored procedures can be used for data analysis and statistics, such as calculating average, median and other statistical indicators.

For example, the following stored procedure uses a WHILE loop to calculate the average price of goods:

CREATE PROCEDURE avg_price()
BEGIN
DECLARE p_price DECIMAL(10,2);
DECLARE total DECIMAL(10,2) DEFAULT 0;
DECLARE count INT DEFAULT 0;
DECLARE done INT DEFAULT FALSE;
DECLARE cur CURSOR FOR SELECT price FROM product;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
OPEN cur;
REPEAT
FETCH cur INTO p_price;
IF NOT done THEN
SET total = total + p_price;
SET count = count + 1;
END IF;
UNTIL done END REPEAT;
CLOSE cur;
SELECT total/count;
END;

Among them, a cursor cur is defined to obtain the price of goods in the product table and use the loop calculation The sum of the price and the quantity of the item, and finally returns the average price.

3. Precautions for loop use of MySQL stored procedures

When using the loop structure of MySQL stored procedures, you need to pay attention to the following matters:

1. Initial value of loop counter and end values ​​need to be set correctly, otherwise it may result in infinite loops or missing data.
2. In the WHILE loop, you need to manually update the value of the loop counter, otherwise the loop will continue.
3. When using the WHILE loop, the CONTINUE HANDLER FOR NOT FOUND statement needs to be set correctly to prevent the stored procedure from looping infinitely after the cursor traverses to the end.
4. In a FOR loop, you need to use the correct loop counter range, otherwise data may be missed or duplicated.

4. Summary

The loop structure of MySQL stored procedures is an important tool for developers in data processing and analysis. It can be used to efficiently process large amounts of data and calculate indicators. When using a loop structure, you need to pay attention to issues such as the initial value and end value of the loop counter, the correctness of the loop statement, and the use of cursors to ensure the stability and correctness of the stored procedure.

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