[MySQL] basic use of cursor

1. What is a cursor （ Or the cursor ）

• Although it can be filtered by WHERE and HAVING, Or a keyword that limits the return record LIMIT Return a record , however , But it can't be like a pointer in the result set , Position a record forward 、 Position a record backward , Or is it Randomly locate a record , And process the recorded data .

• This is the time , You can use a cursor . The cursor , It provides a flexible operation mode , Be able to locate each record in the result set , A data structure that operates on the data in the pointed record . Cursor let SQL This set oriented language has the ability of process oriented development .

• stay SQL in , A cursor is a temporary database object , Can point to the data row pointer stored in the database table . Here's the cursor Acts as a pointer , You can manipulate data rows by manipulating cursors .

• MySQL Cursors can be used in stored procedures and functions .

such as , The query employees The salary in the data sheet is higher than 15000 What are your employees ：

SELECT employee_id,last_name,salary FROM employees
WHERE salary > 15000;

Here, you can operate data rows through cursors , As shown in the figure, the line where the cursor is located is “108” The record of , You can also scroll the cursor on the result set , Point to any row in the result set .

2. Using cursor steps

• The cursor must be declared before the handler is declared , And variables and conditions must also be declared before declaring cursors or handlers .

• If you want to use a cursor , It usually takes four steps . Different DBMS in , The syntax for using cursors may be slightly different .

First step , declare cursor

stay MySQL in , Use DECLARE Keyword to declare the cursor , The basic form of its grammar is as follows ：

DECLARE cursor_name CURSOR FOR select_statement; 

This grammar applies to MySQL,SQL Server,DB2 and MariaDB. If it is to use Oracle perhaps PostgreSQL, Need to be written ：

DECLARE cursor_name CURSOR IS select_statement;

To use SELECT Statement to get the data result set , At this time, the traversal of the data has not started , here select_statement It stands for SELECT sentence , Returns a result set used to create the cursor .

such as ：

DECLARE cur_emp CURSOR FOR 
SELECT employee_id,salary FROM employees;

DECLARE cursor_fruit CURSOR FOR 
SELECT f_name, f_price FROM fruits ;

The second step , Open cursor

The syntax for opening a cursor is as follows ：

OPEN cursor_name

When we define the cursor , If you want to use a cursor , The cursor must be opened first . When opening the cursor SELECT The query result set of the statement will be sent to the cursor workspace , For the following cursor Read one by one Prepare the records in the result set .

OPEN　cur_emp;

The third step , Use cursors （ Get data from cursor ）

The grammar is as follows ：

FETCH cursor_name INTO var_name [, var_name] ...

The function of this sentence is to use cursor_name Use this cursor to read the current row , And save the data to var_name In this variable , The cursor pointer points to the next line . If the data row read by the cursor has multiple column names , It's in INTO Assign a value to multiple variable names after the keyword .

Be careful ：var_name The cursor must be defined before it is declared .

FETCH　cur_emp INTO emp_id, emp_sal ;

Be careful ： The number of fields in the query result set of the cursor , Must follow INTO The following variables are the same , otherwise , When the stored procedure executes ,MySQL It will give you an error .

Step four , Close cursor

CLOSE cursor_name

Yes OPEN There will be CLOSE, That is, open and close the cursor . Close the cursor after using it . Because the cursor will Occupy system resources , If you don't close it in time , The cursor remains until the end of the stored procedure , Affect the efficiency of system operation . And closing the cursor , The system resources occupied by the cursor will be released .

After closing the cursor , You can no longer retrieve the data rows in the query results , If you need to retrieve, you can only open the cursor again .

CLOSE　cur_emp;

3. give an example

• Create stored procedure “get_count_by_limit_total_salary()”, Statement IN Parameters limit_total_salary,DOUBLE type ; Statement OUT Parameters total_count,INT type . The function can accumulate the salary values of several employees with the highest salary , Until the total salary reaches limit_total_salary The value of the parameter , Return the accumulated number of people to total_count.

DELIMITER //

CREATE PROCEDURE get_count_by_limit_total_salary(IN limit_total_salary DOUBLE,OUT total_count INT)

BEGIN
	DECLARE sum_salary DOUBLE DEFAULT 0;  # Record the accumulated total salary 
	DECLARE cursor_salary DOUBLE DEFAULT 0; # Record a salary value 
	DECLARE emp_count INT DEFAULT 0; # Record the number of cycles 
	# Define cursors 
	DECLARE emp_cursor CURSOR FOR SELECT salary FROM employees ORDER BY salary DESC;
	# Open cursor 
	OPEN emp_cursor;
	
	REPEAT
		# Use cursors （ Get data from cursor ）
		FETCH emp_cursor INTO cursor_salary;
		
		SET sum_salary = sum_salary + cursor_salary;
		SET emp_count = emp_count + 1;
		
		UNTIL sum_salary >= limit_total_salary
	END REPEAT;
	
	SET total_count = emp_count;
	# Close cursor 
	CLOSE emp_cursor;
	
END //

DELIMITER ;

• Inquire about How many individuals' wages are taken online more than 20w

# call 
CALL get_count_by_limit_total_salary(200000,@total_count);
SELECT @total_count;

4. Summary

• The cursor is MySQL An important function of , by Read one by one The data in the result set , Provides the perfect solution . Compared with implementing the same function at the application level , Cursors can be used in stored programs , Efficient , The program is also more concise .

• But it also brings some performance problems , For example, in the process of using cursors , Data rows will be Lock , In this way, when there is a large amount of business concurrency , It will not only affect the efficiency between businesses , Will Consume system resources , Cause insufficient memory , This is because cursors are processed in memory .

• Suggest ： The habit of closing after use , This can improve the overall efficiency of the system .