On cursor in MySQL

What is a cursor ？

The cursor （cursor） Is a store in MySQL Database query on server , It's not one SELECT sentence , But the result set retrieved by the statement . In the store After bid , The application can scroll or browse the data as needed .

Be careful ：MySQL Cursors can be used for stored procedure , function , trigger , Incident

Cursor properties

• The database can also choose not to copy the result set
• Not updatable
• The cursor can only travel in one direction , And you can't skip any line of data . To use a cursor , First define a cursor variable

Create cursors

Before creating a cursor , We need to clear the syntax of the cursor first

1、 Define cursors

DECLARE Cursor name CURSOR FOR SQL sentence ;

2、 Open cursor

OPEN Cursor name ;

3、 To get the results

FETCH Cursor name INTO Variable name [, Variable name ];

4、 Close cursor

CLOSE Cursor name ;

We use Customers Table as an example

Example 1

Define a stored procedure , When called, execute the cursor inside

CREATE PROCEDURE PROC1()
BEGIN
    --  Define two variables to store the results 
    DECLARE NAME VARCHAR(20);
    DECLARE ADDR VARCHAR(50);
    --  declare cursor 
    DECLARE MY CURSOR FOR SELECT  full name , Address  FROM customers;
    --  Open cursor 
    OPEN MY;
    --  To get the results 
    FETCH MY INTO NAME,ADDR;
    --  Here is to show the obtained results 
    SELECT NAME,ADDR;
    --  Close cursor 
    CLOSE MY;    
END;

After we execute the above stored procedure , You can call the stored procedure

CALL PROC1();

Get the results ：

There must be some friends here who are curious ,customers It's on the outside 7 Bar record , Why only 1 Bar record ？

This is because the variables of the cursor only retain customers The first row of data in the table , If you want to view the following data , You need to cycle down the cursor , To continue viewing .

Example 2

Define a stored procedure , When calling a stored procedure , Will table customers The data in the table is written to the new table .

CREATE PROCEDURE PROC2()
BEGIN
    --  Define two variables to store the results 
    DECLARE FLAG INT DEFAULT 0; 
    DECLARE NAME VARCHAR(20);
    DECLARE ADDR VARCHAR(50);
    --  declare cursor 
    DECLARE MY CURSOR FOR SELECT  full name , Address  FROM customers;

    DECLARE CONTINUE HANDLER FOR NOT FOUND SET FLAG=1;
    --  Open cursor 
    OPEN MY;
    --  Circulating body part 
    L1:LOOP
    --  To get the results 
    FETCH MY INTO NAME,ADDR;
    IF FLAG=1 THEN
       LEAVE L1;
    END IF;
    --  Here is to show the obtained results 
    INSERT INTO cus VALUES(NAME,ADDR);
    --  Close cursor 
    END LOOP;  --  End of cycle 
    CLOSE MY;    
END;

Then we execute the stored procedure , And query cus Data in table

CALL PROC2();SELECT * FROM cus;

result ：

Results and customers The same in , But these results are inserted in the process of moving down one by one , That is, the loop executes 7 Time .

These are the basic operating principles of cursors , In addition, the loop body of the cursor has WHILE,REPEAT Etc , They operate in the same way LOOP similar , Are used to loop the contents of the loop body , Until the end of the cycle .

Use scenarios

When we go through sql When querying data ,

1, The common way is to sql In the past , The server returns all the data to you , Another way is

2, Cursor mode , The cursor method will find the data to be queried on the server side , Then return it to you in batches , This method is suitable for scenarios where a large amount of operation data is to be operated , The server gave me a message , I deal with one , Then until the end of the loop processing , client You won't receive all the data at once

In the first way, there will be memory leakage ,

The second is not , But it takes up server links for a long time

Cursors are simply understood as ： Tell the server the data result set I want to query , Then return to me in batches and deal with it slowly

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