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我正在尝试创建一个随机样本（30-70），如下所示：

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

# Post this 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，在这里将两个条件结合起来时，它不起作用。看起来带有 predict_var ='2' 或 predict_var ='4' 或带有 '' 或带有 "" 的 where 条件不起作用 - 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（我认为这是非常低效的，因为这会创建临时表（我也可以选择视图，但重点是让它正确）

# 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/

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1 回答 1

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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。

于 2014-01-20T15:31:40.690 回答