What is the function of MySQL cursors

mysql What is the function of a cursor

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stay mysql in , Cursor has the function of pointer , It is mainly used to traverse the record result set returned by the query database , In order to carry out the corresponding operation . A cursor is actually a mechanism that can extract one record at a time from a result set that contains multiple data records . The essence of RDBMS is set oriented , stay MySQL There is no representation in the table that describes a single record in the table , Unless used WHERE Clause to restrict only one record to be selected ; So sometimes we must use the cursor to process the data of a single record .

The operating environment of this tutorial ：windows7 System 、mysql8 edition 、Dell G3 The computer .

MySQL The cursor （Cursor）

A cursor is actually a mechanism that can extract one record at a time from a result set that contains multiple data records .

The cursor acts as a pointer .

Although the cursor can traverse all the rows in the result , But he only points to one line at a time .

The function of cursor is to traverse the records returned by query database , In order to carry out the corresponding operation .

The essence of RDBMS is set oriented , stay MySQL There is no representation in the table that describes a single record in the table , Unless used WHERE Clause to restrict only one record to be selected . So sometimes we must use the cursor to process the data of a single record .

Generally, data modification is performed by locating the cursor to a row of the result set .

Usage of cursors

1、 Declare a cursor ： declare Cursor name CURSOR for table;( there table It can be any set you query out )

2、 Open defined cursor ：open Cursor name ;

3、 Get the next row of data ：FETCH   Cursor name into testrangeid,versionid;

4、 Statements that need to be executed ( Additions and deletions )： It depends on the circumstances

5、 Release cursor ：CLOSE Cursor name ;

notes :mysql The stored procedure must be followed by ; ending , The temporary fields used need to be declared before the cursor is defined .

example

-  BEGIN    -- Defining variables   declare testrangeid BIGINT;  declare versionid BIGINT;   declare done int;  -- Create cursors , And store data   declare cur_test CURSOR for      select id as testrangeid,version_id as versionid from tp_testrange;  -- After the contents of the cursor are executed, the done Set to 1   DECLARE CONTINUE HANDLER FOR NOT FOUND SET done=1;   -- Open cursor   open cur_test;  -- Execute loop     posLoop:LOOP  -- Determine whether to end the loop           IF done=1 THEN          LEAVE posLoop;      END IF;   -- Take the value in the cursor       FETCH  cur_test into testrangeid,versionid;  -- Perform an update operation       update tp_data_execute set version_id=versionid where testrange_id = testrangeid;    END LOOP posLoop;  -- Release cursor   CLOSE cur_test;    END  -

Example 2：

Now we're going to do a function with stored procedures , Statistics iphone What's the total stock of , And output the total to the console .

-- stay windows When writing stored procedures in the system , If needed declare Declare variables , You need to add this keyword , Otherwise, an error will be reported .  delimiter //  drop procedure if exists StatisticStore;  CREATE PROCEDURE StatisticStore()  BEGIN      -- Create variables to receive cursor data       declare c int;      declare n varchar(20);      -- Create a total variable       declare total int default 0;      -- Create an end flag variable       declare done int default false;      -- Create cursors       declare cur cursor for select name,count from store where name = 'iphone';      -- Specifies the return value at the end of the cursor loop       declare continue HANDLER for not found set done = true;      -- Set initial value       set total = 0;      -- Open cursor       open cur;      -- Start looping through the data in the cursor       read_loop:loop      -- According to a piece of data that the cursor currently points to       fetch cur into n,c;      -- Determine whether the cursor's loop ends       if done then          leave read_loop;    -- Jump out of the cursor loop       end if;      -- When getting a piece of data , take count Value to be accumulated , Here you can do whatever you want ,      set total = total + c;      -- End cursor loop       end loop;      -- Close cursor       close cur;        -- Output results       select total;  END;  -- Calling stored procedure   call StatisticStore();

fetch Is to get the data row that the cursor currently points to , And point the pointer to the next line , Continuing to execute when the cursor has already pointed to the last row will cause cursor overflow .
Use loop When you cycle the cursor , He doesn't monitor the last data himself , Like the following code , It's going to create a dead cycle ;

read_loop:loop  fetch cur into n,c;  set total = total+c;  end loop;

stay MySql in , Causes a cursor overflow mysql Predefined NOT FOUND error , So use the following code above to specify when to raise not found Define a... When it's wrong continue Events , Specifies to modify when this event occurs done The value of the variable .

declare continue HANDLER for not found set done = true;

So I added the following code in the loop ：

-- Determine whether the cursor's loop ends   if done then      leave read_loop;    -- Jump out of the cursor loop   end if;

If done The value of is true, Just close the loop . Continue with the following code

Usage mode

There are three ways to use cursors ：

• The first is the above implementation , Use loop loop ;

• The second way is as follows , Use while loop ：

drop procedure if exists StatisticStore1;  CREATE PROCEDURE StatisticStore1()  BEGIN      declare c int;      declare n varchar(20);      declare total int default 0;      declare done int default false;      declare cur cursor for select name,count from store where name = 'iphone';      declare continue HANDLER for not found set done = true;      set total = 0;      open cur;      fetch cur into n,c;      while(not done) do          set total = total + c;          fetch cur into n,c;      end while;            close cur;      select total;  END;    call StatisticStore1();

The third way is to use repeat perform ：

drop procedure if exists StatisticStore2;  CREATE PROCEDURE StatisticStore2()  BEGIN      declare c int;      declare n varchar(20);      declare total int default 0;      declare done int default false;      declare cur cursor for select name,count from store where name = 'iphone';      declare continue HANDLER for not found set done = true;      set total = 0;      open cur;      repeat      fetch cur into n,c;      if not done then          set total = total + c;      end if;      until done end repeat;      close cur;      select total;  END;    call StatisticStore2();

Cursor nesting

stay mysql in , Every begin end The blocks are all independent scope Area , because MySql The same one error Can only be defined once , If there are multiple definitions, you will be prompted at compile time Duplicate handler declared in the same block.

drop procedure if exists StatisticStore3;  CREATE PROCEDURE StatisticStore3()  BEGIN      declare _n varchar(20);      declare done int default false;      declare cur cursor for select name from store group by name;      declare continue HANDLER for not found set done = true;      open cur;      read_loop:loop      fetch cur into _n;      if done then          leave read_loop;      end if;      begin          declare c int;          declare n varchar(20);          declare total int default 0;          declare done int default false;          declare cur cursor for select name,count from store where name = 'iphone';          declare continue HANDLER for not found set done = true;          set total = 0;          open cur;          iphone_loop:loop          fetch cur into n,c;          if done then              leave iphone_loop;          end if;          set total = total + c;          end loop;          close cur;          select _n,n,total;      end;      begin              declare c int;              declare n varchar(20);              declare total int default 0;              declare done int default false;              declare cur cursor for select name,count from store where name = 'android';              declare continue HANDLER for not found set done = true;              set total = 0;              open cur;              android_loop:loop              fetch cur into n,c;              if done then                  leave android_loop;              end if;              set total = total + c;              end loop;              close cur;          select _n,n,total;      end;      begin            end;      end loop;      close cur;  END;    call StatisticStore3();

The above is to implement a nested loop , Of course, this example is far fetched . Just make do with it .

dynamic SQL

Mysql Support dynamic SQL The function of

set @sqlStr='select * from table where condition1 = ?';  prepare s1 for @sqlStr;  -- If there are multiple arguments separated by commas   execute s1 using @condition1;  -- Release by hand , Or is it  connection  closed , server  Automatic recovery   deallocate prepare s1;

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