Question

我尝试使用UNION在一个数据库中选择3个表中的记录，但我发现了错误：

#1248 - 每个派生表都必须有自己的别名

这是我正在尝试的查询：

SELECT * from
  (SELECT column1, 'table1' from table1
UNION
   SELECT column1, 'table2'  from table2
UNION
   SELECT column1, 'table3'  from table3) 
WHERE column1 not like 'abr%' and length(column1) < 8;

错误是什么意思，我该如何解决并显示正确的结果？

Answer 1

SELECT Alis.column1 
  FROM (SELECT column1, 'table1' as which from table1
  UNION ALL
      SELECT column1, 'table2' from table2
  UNION ALL
      SELECT column1, 'table3' from table3
 ) Alis
WHERE column1 not like 'abr%' and length(column1) < 8;

Answer 2

您必须在内部选择或派生表中为您的列添加别名，也为自己添加别名，如下所示：

SELECT * 
FROM
    (SELECT column1, 'table1' as t from table1
     UNION ALL
     SELECT column1, 'table2' as t from table2
     UNION ALL
     SELECT column1, 'table3' as t from table3
    ) As dt
WHERE 
    column1 NOT LIKE 'abr%' 
  AND 
    length(column1) < 8;

已编辑：
我将UNION更改为UNION ALL作为效果问题，并且不会有任何重复;）。

Answer 3

您需要为派生表提供一个别名，如：

SELECT * from
  (SELECT column1, 'table1' from table1
UNION
   SELECT column1, 'table2'  from table2
UNION
   SELECT column1, 'table3'  from table3) dtAlias
WHERE column1 not like 'abr%' and length(column1) < 8;

Answer 4

正如其他人所说，问题很简单 - 在最后一个括号后需要一个表别名。您还需要第二列的列别名。

出于性能原因，更重要的是您应该使用UNION ALL。 UNION会产生删除重复项的开销，在这种情况下完全没有必要：

SELECT t.* 
FROM (SELECT column1, 'table1' as which from table1
      UNION ALL
      SELECT column1, 'table2' from table2
      UNION ALL
      SELECT column1, 'table3' from table3
     ) t
WHERE column1 not like 'abr%' and length(column1) < 8;

对于MySQL中的性能，我认为您希望将WHERE子句移动到子查询，并且在所有三个表中的column1上创建索引：

SELECT t.* 
FROM (SELECT column1, 'table1' as which from table1 WHERE column1 not like 'abr%' and length(column1) < 8
      UNION ALL
      SELECT column1, 'table2' from table2 WHERE column1 not like 'abr%' and length(column1) < 8
      UNION ALL
      SELECT column1, 'table3' from table3 WHERE column1 not like 'abr%' and length(column1) < 8
     ) t

Answer 5

你应该为派生表命名，如下所示

SELECT * from
  (SELECT column1, 'table1' from table1
UNION
   SELECT column1, 'table2'  from table2
UNION
   SELECT column1, 'table3'  from table3) as new_table
WHERE column1 not like 'abr%' and length(column1) < 8;

Answer 6

错误非常自我解释，您只需要使用嵌套选择和联合命名您创建的派生表。添加别名来修复我们的错误应该足够了，如下所示：

  SELECT * from
  (SELECT column1, 'table1' from table1
   UNION
   SELECT column1, 'table2'  from table2
   UNION
   SELECT column1, 'table3'  from table3) as youralias 
   WHERE column1 not like 'abr%' and length(column1) < 8;

Answer 7

您应该在子查询中使用的表中添加别名。这是正确的版本：

USE test2;

SELECT * from
  (SELECT column1, 'table1' from table1 as t1
UNION
   SELECT column1, 'table2'  from table2 as t2
UNION
   SELECT column1, 'table3'  from table3) as t3
WHERE column1 not like 'abr%' and length(column1) < 8;

使用UNION从多个表中进行选择

7 个答案: