mysql中使用局部变量和随机化的limit union和order by子句的正确语法：错误（1221）答案

【问题标题】：correct syntax for limit union and order by clause with local variables and randomization in mysql : Error(1221)mysql中使用局部变量和随机化的limit union和order by子句的正确语法：错误（1221）
【发布时间】：2014-02-09 20:18:16
【问题描述】：

我正在尝试创建如下随机样本 (30-70) -:

set @total=(select count(*) from tab_1 where predict_var ="4" or predict_var ="2" ) ;
set @sample= (select @total*(70/30))  ;

#@total,@sample
#4090,9543.333331970

# 发布这个我可以通过这个通用语法获取元素

PREPARE STMT FROM "SELECT * FROM tab_1 where predict_var ='2' or  predict_var ='4' ORDER BY RAND() LIMIT ? " ;
EXECUTE STMT USING @sample; # **THIS WORKS**

但是，当将两个条件的并集作为此处时，它不起作用。它看起来像 where 条件与 predict_var ='2' 或 predict_var ='4' 或与 ''或使用 "" 不起作用 - 我该如何使它起作用？

我的完整用例是这样的 -:

#This logic is my end-goal in retrieval of records
PREPARE STMT FROM " SELECT * FROM tab_1 WHERE predict_var = '4' or predict_var = '2' ORDER BY RAND() union (SELECT * FROM tab_1 WHERE predict_var = '0' or predict_var = '1' ORDER BY RAND() limit ?  )"
EXECUTE STMT USING @sample; # **does not work**

# This does not work either and gives "Error(1221) incorrect usage of union and order by

我也尝试过这样的事情（我认为这非常低效，因为这会创建临时表（我也可以选择视图，但重点是正确）

# using temp table/ could also get it via views - but doesn't work,nevertheless
#Alternate logic - but does not work either

PREPARE STMT FROM "SELECT * FROM tab_1 WHERE predict_var = '0' or predict_var = '1' ORDER BY RAND() limit ? " ; # Works

create table temp
select * from (EXECUTE STMT USING @sample ) ; # Error(1064)

SELECT * FROM tab_1 WHERE predict_var = '4' or predict_var = '2' ORDER BY  RAND() union select * from temp ; # of course does not work

我隐约明白这是关于移动 union & order by 子句提示：- 请解释为什么会失败，如果可能的话，不要建议其他方法，比如连接。相关 - http://orkus.wordpress.com/2010/06/29/variable-limit-statement-in-mysql/

【问题讨论】：

标签： mysql union where-clause random-sample sql-limit

【解决方案1】：

看来我明白了！

PREPARE STMT FROM " SELECT * FROM tab_1 WHERE predict_var = '4' or predict_var = '2'  union 
(SELECT * FROM tab_1 WHERE predict_var = '0' or predict_var = '1' limit ?  ) ORDER BY RAND() " ;
EXECUTE STMT USING @sample; # fetches a total of 13,633 records

*我唯一改变的是从第一个查询中完全删除 order by 并将其放在组合查询之后（真的是小混混！）。如果这不起作用，随机化明智 - 或者如果您有更好的建议，请轻推。

【讨论】：