Question

I’ve a query that I need some help with –
As part of a form I’ve got a serial number field that is populated if there is a serial number, blank if it’s not, or no result if it’s an invalid serial number.

select * 
from cust_site_contract as cs 
where cs.serial_no = 'C20050' or (cs.serial_no <> 'C20050' and if(cs.serial_no = 'C20050',1,0)=0)
limit 10;

Here’s a sample of the regular data:

+----------------------+-----------+-----------+-----------
| idcust_site_contract | system_id | serial_no | end_date
+----------------------+-----------+-----------+-----------
|               561315 | SH001626  | C19244    | 2009-12-21
|               561316 | SH001626  | C19244    | 2010-06-30
|               561317 | SH002125  | C19671    | 2010-05-31
|               561318 | SH001766  | C14781    | 2010-09-25
|               561319 | SH001766  | C14781    | 2011-02-15
|               561320 | SH002059  | C19020    | 2008-07-09
|               561321 | SH002639  | C18889    | 2008-03-31
|               561322 | SH002639  | C18889    | 2008-06-30
|               561323 | SH002715  | C20051    | 2010-04-30
|               561324 | SH002719  | C20057    | 2010-04-30

And an exact result would look something like this:

|               561487 | SH002837  | C20050    | 2012-07-04

I was writing this as a subquery so I could match the system_ids to customer and contract names, but realised I was getting garbage pretty early on.
I’m tempted to try and simplify it by saying the third case might not hold true (i.e. if it’s an invalid serial number, allow the choice of any customer name and simply flag it in the data)
Has anyone got any ideas of where I’m going wrong? The combination of conditions is clearly wrong, and I can’t work out how to make each side of the or statement mutually exclusive
Even if I try to evaluate only the if(sn = ‘blah’) I get the wrong result for obvious reasons, but can’t think of a sane way to express it.
Many thanks
Scott

Answer 1

If there is is no contract with a serial number of C20050, this query will return all rows, otherwise, it will return only one row where serial_no is C20050:

SELECT a.*
FROM cust_site_contract a
INNER JOIN
(
    SELECT COUNT(*) AS rowexists
    FROM cust_site_contract
    WHERE serial_no = 'C20050'
) b ON b.rowexists = 0

UNION ALL

(
    SELECT *
    FROM cust_site_contract 
    WHERE serial_no = 'C20050'
    LIMIT 1
)