The difference between stored procedures and functions is their purpose and call method: 1. Stored procedures can have multiple input and output parameters or no parameters, and are called with CALL; the function must have a return value and can only have one RETURNS value, which can be called in SELECT. 2. Before writing the storage structure, you need to use DELIMITER to replace the ending character such as // or $$ to avoid parsing the semicolon in advance, and restore the default separator after writing. 3. The variable declaration should be placed before all statements, use DECLARE to define local variables, and pay attention to the correct format of process control syntax such as IF, CASE, LOOP, and WHILE. 4. Debugging can be inserted into debugging information by log tables. It is recommended to add comments to explain the functions and parameters meanings, keep the logic clear, clean redundant objects regularly, and improve maintenance efficiency.
Writing MySQL stored procedures and functions is actually a practical skill, especially when dealing with complex business logic or requiring a reduction in the number of interactions between databases and applications. Although many people may feel a little confused at first, once they master the basic routines, they are quite easy to use.
1. First understand the difference between stored procedures and functions
The stored procedures (Procedures) and functions (Functions) in MySQL look similar, but there are still some differences in uses:
- Stored procedure : There can be multiple input and output parameters, or even no parameters. Use
CALLstatements when executing. - Function : There must be a return value, only one
RETURNSvalue, and the call method is more like an expression, such as calling directly inSELECT.
Let's give a simple example:
-- Function example: Return the discounted price of a certain order DELIMITER //
CREATE FUNCTION get_discounted_price(price DECIMAL(10,2), discount DECIMAL(5,2))
RETURNS DECIMAL(10,2)
BEGIN
RETURN price * (1 - discount);
END //
DELIMITER ;
-- Call the function SELECT get_discounted_price(100, 0.1); -- Return 902. Use DELIMITER to change the ending character of the statement
When writing a storage structure, it may contain semicolons ( ; ), which will cause MySQL to parse your statements in advance. At this time, you need to use DELIMITER to replace a temporary ending symbol, such as // or $$ .
Common steps are as follows:
- Execute
DELIMITER // - Write your storage structure
- Finally, change it back to
DELIMITER ;
Note that don't forget to restore the default delimiter at the end, otherwise subsequent SQL may error.
3. Be careful when declaring variables and process control
In stored procedures, the order and scope of variables are important. Local variables need to be declared with DECLARE and must be preceded by all other statements.
For example:
DELIMITER //
CREATE PROCEDURE calculate_total(IN order_id INT, OUT total DECIMAL(10,2))
BEGIN
DECLARE subtotal DECIMAL(10,2);
DECLARE shipping DECIMAL(10,2);
SELECT SUM(price * quantity) INTO subtotal
FROM order_items
WHERE order_id = order_id;
SET shipping = 10.00;
SET total = subtotal shipping;
END //
DELIMITER ; In addition, process control statements such as IF , CASE , LOOP , and WHILE can all be used in it, but pay attention to the syntax format, especially the nesting of BEGIN ... END .
4. Debugging and maintenance are key difficulties
After writing stored procedures or functions, debugging is a headache. MySQL does not have a built-in debugger, so you may have to print logs to troubleshoot problems.
A common practice is to create a log table and then insert debug information at a critical location, such as:
INSERT INTO debug_log (message, created_at) VALUES ('Processing order', NOW());
Of course, this method is relatively primitive, but it is very effective for troubleshooting process errors.
There are a few more suggestions:
- Add comments to each stored procedure to indicate the function and parameter meaning
- Avoid stuffing too complex logic into the database and keep responsibilities clear
- Regularly check whether there are redundant or unused storage objects and clean them up in time
Basically that's it. When you first write, you are likely to get stuck in the grammatical details, and practicing a few more times will make you feel better.
The above is the detailed content of Writing stored procedures and functions in MySQL. For more information, please follow other related articles on the PHP Chinese website!
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