如何在SQL中使用空的虚拟列过滤查询？

Question

I have situation where must compare empty dummy field in like .

SELECT id, name, contact, landline, '' fax
FROM table1 
WHERE contact LIKE '%123%'
   OR landline LIKE '%123%'
   OR fax LIKE '%123%'

After execution it gives error like:

Unknown column 'fax' in 'where clause'

Is it possible?

Answer 1

This is not possible; you cannot use aliases defined in the same SELECT in the WHERE .

MySQL extends the use of the HAVING clause, so you can move the conditions to HAVING :

SELECT id, name, contact, landline, '' as fax
FROM table1
HAVING contact LIKE '%123%' OR landline LIKE '%123%' OR fax LIKE '%123%'

Answer 2

Wrap the original query up in a derived table (sub-query). Apply the conditions on its result:

select id, name, contact, landline, fax
from
(
    SELECT id, name, contact, landline, '' fax
    FROM table1 
) dt
WHERE contact LIKE '%123%'
   OR landline LIKE '%123%'
   OR fax LIKE '%123%'

ANSI SQL compliant solution, will work with most dbms products.