Detailed explanation of the usage and function of MySQL cursor

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BEGIN 
 
--Define variables declare testrangeid BIGINT; 
declare versionid BIGINT; 
declare done int; 
--Create a cursor and store data declare cur_test CURSOR for 
 select id as testrangeid,version_id as versionid from tp_testrange; 
--After the content in the cursor is executed, set done to 1 
 DECLARE CONTINUE HANDLER FOR NOT FOUND SET done=1; 
--Open the cursor open cur_test; 
--Execute loop posLoop:LOOP 
--Judge whether to end the loop IF done=1 THEN 
  LEAVE posLoop; 
 END IF; 
--Get the value in the cursor FETCH cur_test into testrangeid,versionid; 
--Execute update operation update tp_data_execute set version_id=versionid where testrange_id = testrangeid; 
 END LOOP posLoop; 
--Release the cursor CLOSE cur_test; 
 
END 
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--When writing a stored procedure in a Windows system, if you need to use declare to declare a variable, you need to add this keyword, otherwise an error will be reported. 
delimiter // 
drop procedure if exists StatisticStore; 
CREATE PROCEDURE StatisticStore() 
BEGIN 
 --Create a variable to receive cursor data declare c int; 
 declare n varchar(20); 
 --Create total variable declare total int default 0; 
 --Create end flag variable declare done int default false; 
 --Create a cursor declare cur cursor for select name,count from store where name = 'iphone'; 
 --Specify the return value at the end of the cursor loop declare continue HANDLER for not found set done = true; 
 --Set the initial value set total = 0; 
 --Open the cursor open cur; 
 --Start looping over the data in the cursor read_loop:loop 
 --fetch cur into n,c according to the data currently pointed to by the cursor; 
 --Judge whether the cursor loop is finished if done then 
  leave read_loop; --Jump out of cursor loop end if; 
 --When getting a piece of data, add the count value. You can do whatever you want here. 
 set total = total + c; 
 --End the cursor loop end loop; 
 --Close the cursor close cur; 
 
 --Output results select total; 
END; 
--Call the stored procedure call StatisticStore();

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

declare continue HANDLER for not found set done = true;

--Judge whether the cursor loop is finished if done then 
 leave read_loop; --Jump out of cursor loop end if;

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();

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();

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();

set @sqlStr='select * from table where condition1 = ?'; 
prepare s1 for @sqlStr; 
--If there are multiple parameters, separate them with commas execute s1 using @condition1; 
--Manual release, or when the connection is closed, the server automatically recycles deallocate prepare s1;