mysql限制子查詢

Question

我想動態地在我的sql查詢中設置限制。我想在參數表中設置限制。

select col1, col2, col3 
from table 
where col4 = 'abc' 
limit (select a from param) 

如何做到這一點？請幫助

Answer 1

您不能使用參數limit（除非您有準備好的語句）。您可以使用變量來枚舉行：

select col1, col2, col3 
from (select t.*, (@rn := @rn + 1) as rn 
     from table cross join 
      (select @rn := 0) const 
    ) t 
where rn <= PARAMETERVALUE; 

雖然我用這個子查詢，我想你也可以這樣做：

select col1, col2, col3 
from table t cross join 
    (select @rn := 0) const 
where (@rn := @rn + 1) <= PARAMETERVALUE; 

Answer 2

您可以使用PREPARED STATEMENTS

mysql> select * from t; 
+----+ 
| id | 
+----+ 
| 1 | 
| 2 | 
| 3 | 
| 4 | 
| 5 | 
+----+ 
5 rows in set (0.00 sec) 

mysql> prepare stmt from 'select * from t limit ?'; 
Query OK, 0 rows affected (0.01 sec) 
Statement prepared 

mysql> set @v = 2; 
Query OK, 0 rows affected (0.00 sec) 

mysql> execute stmt using @v; 
+----+ 
| id | 
+----+ 
| 1 | 
| 2 | 
+----+ 
2 rows in set (0.00 sec) 

mysql> deallocate prepare stmt; 
Query OK, 0 rows affected (0.00 sec)