mysql中带有局部变量和随机化的limit union和order by子句的正确语法：错误（1221）

我试图创建一个随机样本（30-70），如下-：

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

#发布本文后，我可以通过这种通用语法获取元素

但是，将这两个条件的并集视为此处的并集时，它不起作用。而且使用“预测”或“预测”或“预测”的where条件（使用“”或“”）似乎不起作用-如何使其起作用

我的完整用例如下-：

#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

我也尝试过类似的方法（我认为这在efficienet中非常有效，因为这将创建临时表（我也可以选择视图，但关键是要正确）

我模模糊糊地理解这是关于移动工会和订单条款 提示：-请解释为什么失败，如果可能的话，不要建议其他方法，比如连接。相关-

看起来我明白了

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（真是太小了！）

# 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 

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