别名在ORDER BY子句中不起作用

Question

This will work fine in MySQL 5:

SELECT INSTR(foo, 'Bar') as foobar
FROM Table
ORDER BY CASE WHEN foobar = 1 THEN foo END DESC;

Bu in MySQL 4, I get the error:

ERROR 1054 (42S22): Unknown column 'foobar' in 'order clause'

However, if I change the clause to this, it will work on both versions:

SELECT INSTR(foo, 'Bar') as foobar
FROM Table
ORDER BY CASE WHEN INSTR(foo, 'Bar') = 1 THEN foo END DESC;

To ensure compatibility, do I have to always use the second way?

Answer 1

because foobar is an alias. it is not a column unless it's from derived query ( subquery ).like thisone below

SELECT * 
FROM
(
   SELECT INSTR(foo, 'Bar') as foobar
   FROM Table
) a
ORDER BY CASE WHEN foobar = 1 THEN foo END DESC;

Answer 2

Basically you cannot re-use a column alias at the same "query-level".

If you want to be write compatible/portable SQL and don't want to repeat the function call, then the usual way to do it, is to wrap the query into a derived table.

select *
from (
  SELECT INSTR(foo, 'Bar') as foobar,
         foo
  FROM Table
) t
ORDER BY CASE WHEN foobar = 1 THEN foo END DESC;

Note that you need to include any column you want to use in the "outer" query inside the inner query. If you omit the column foo in the inner derived table, you can not access it on the outer level.

Answer 3

As per other answers, you can not use alias in CASE .
Instead of using sub-query you can direct use INSTR() in CASE like this:

SELECT INSTR(foo, 'Bar') as foobar
FROM Table
ORDER BY CASE WHEN INSTR(foo, 'Bar') = 1 THEN foo END DESC;

When you are using sub-query then note that you will also have to select foo column to order by it, other wise you will get an error like this

So your query with sub-query should be:

SELECT * FROM
(
    SELECT foo,INSTR(foo, 'Bar') as foobar
    FROM t
) A
ORDER BY CASE WHEN foobar = 1 THEN foo END DESC;