Using variable in a LIMIT clause in MySQL

问题

I am writing a stored procedure where I have an input parameter called my_size that is an INTEGER. I want to be able to use it in a LIMIT clause in a SELECT statement. Apparently this is not supported, is there a way to work around this?

# I want something like:
SELECT * FROM some_table LIMIT my_size;

# Instead of hardcoding a permanent limit:
SELECT * FROM some_table LIMIT 100;

回答1:

A search turned up this article. I've pasted the relevant text below.

Here's a forum post showing an example of prepared statements letting you assign a variable value to the limit clause:

http://forums.mysql.com/read.php?98,126379,133966#msg-133966

However, I think this bug should get some attention because I can't imagine that prepared statements within a procedure will allow for any procedure-compile-time optimizations. I have a feeling that prepared statements are compiled and executed at the runtime of the procedure, which probaby has a negative impact on efficiency. If the limit clause could accept normal procedure variables (say, a procedure argument), then the database could still perform compile-time optimizations on the rest of the query, within the procedure. This would likely yield faster execution of the procedure. I'm no expert though.

回答2:

For those, who cannot use MySQL 5.5.6+ and don't want to write a stored procedure, there is another variant. We can add where clause on a subselect with ROWNUM.

SET @limit = 10;
SELECT * FROM (
  SELECT instances.*, 
         @rownum := @rownum + 1 AS rank
    FROM instances, 
         (SELECT @rownum := 0) r
) d WHERE rank < @limit;

回答3:

STORED PROCEDURE

DELIMITER $
create PROCEDURE get_users(page_from INT, page_size INT)
begin
SET @_page_from = page_from;
SET @_page_size = page_size;
PREPARE stmt FROM "select u.user_id, u.firstname, u.lastname from users u limit ?, ?;";
EXECUTE stmt USING @_page_from, @_page_size;
DEALLOCATE PREPARE stmt;
end$
DELIMITER ;

USAGE

call get_users(1, 10);

回答4:

I know this answer has come late, but try SQL_SELECT_LIMIT.

Example:

Declare rowCount int;
Set rowCount = 100;
Set SQL_SELECT_LIMIT = rowCount;
Select blah blah
Set SQL_SELECT_LIMIT = Default;

回答5:

This feature has been added to MySQL 5.5.6. Check this link out.

I've upgraded to MySQL 5.5 just for this feature and works great. 5.5 also has a lot of performance upgrades in place and I totally recommend it.

回答6:

Another way, the same as wrote "Pradeep Sanjaya", but using CONCAT:

CREATE PROCEDURE `some_func`(startIndex INT, countNum INT)
READS SQL DATA
  COMMENT 'example'
BEGIN
  SET @asd = CONCAT('SELECT `id` FROM `table` LIMIT ',startIndex,',',countNum);
  PREPARE zxc FROM @asd;
  EXECUTE zxc;
END;

回答7:

As of MySQL version 5.5.6, you can specify LIMIT and OFFSET with variables / parameters.

For reference, see the 5.5 Manual, the 5.6 Manual and @Quassnoi's answer

来源：https://stackoverflow.com/questions/245180/using-variable-in-a-limit-clause-in-mysql

标签

mysql

variables

limit

clause