Is it Possible to Use IF/Else in SQL? - sql

Question

Is it possible to use if/else in SQL? If I have a table called supplier with columns: sid -> primary key, sname and city.
Then I wish to:
select sid from supplier where city="taipei" if not empty.
Or select sid from supplier where city="tainan"

Answer 1

Yes, you can. I don't know about other DBMS but I have used such things in Microsot SQL Server in my Stored Procedure like this;
IF EXISTS
(SELECT [sid] FROM [supplier] WHERE [city]= "taipei")
select sid from supplier where city="taipei" // your true condition query
ELSE
select sid from supplier where city="tainan"
In MySQL From this link, it turns out that is also possible. see;
IF EXISTS(SELECT * FROM tbl_name WHERE category_code ='some-category-code') THEN UPDATE tbl_name SET active='0' WHERE category_code = 'some-category-code' END IF

Answer 2

It was unclear what you want to do (I leave my previous hypotheses below).
You want to associate a priority to your suppliers, so that the one for Taipei is selected, but if it is unavailable, then Tainan gets selected instead.
In this specific case you can just use:
SELECT sid FROM supplier WHERE city = (
SELECT MAX(city) FROM supplier WHERE city IN ('Taipei', 'Tainan')
);
The inner sub-SELECT will retrieve Taipei or, if unavailable, Tainan.
This uses the fact that Taipei is lexicographically greater than Tainan, but if you wanted a more flexible solution, MAX would not work. In that case you would change the subselect to sort cities in order of desirability (missing cities are of course undesirable) and then fetch the one most desirable:
SELECT sid FROM supplier WHERE city = (
SELECT city FROM supplier ORDER BY CASE
WHEN city = 'Taipei' THEN 1
WHEN city = 'Tainan' THEN 2
WHEN city = 'New York' THEN 3
ELSE 4
END
LIMIT 1
);
The subselect now will retrieve first Taipei, but missing Taipei it will get to Tainan and so on.
Note that if you want only one SID, you can do it much more simply:
SELECT sid FROM supplier ORDER BY CASE
WHEN city = 'Taipei' THEN 1
WHEN city = 'Tainan' THEN 2
WHEN city = 'New York' THEN 3
ELSE 4
END
LIMIT 1
This will retrieve all suppliers, but the one from Taipei, if available, will come out first; and the LIMIT 1 will truncate the response to that first row.
The solutions below do not apply
This will get sid from supplier where city is Taipei or Tainan (which of course means that city is not empty!):
SELECT sid FROM supplier WHERE city IN ('Taipei', 'Tainan');
This will get sid from supplier as above, provided sid is not empty:
SELECT sid FROM supplier WHERE city IN ('Taipei', 'Tainan') AND sid IS NOT NULL;
This will get sid from supplier as above, and replace sid if it is empty.
SELECT CASE WHEN sid IS NULL then 'Empty' ELSE sid END AS sid
FROM supplier WHERE city IN ('Taipei', 'Tainan');
Maybe you should provide two or three sample rows with the expected results.
Edit: sorry, I see now that sid is a primary key, which means it should never be empty. This means that cases 2 and 3 can never apply.
Then perhaps you mean that sname is not empty?:
SELECT sid FROM supplier WHERE city IN ('Taipei', 'Tainan')
AND sname IS NOT NULL AND sname != '';

Answer 3

The following selects a supplier if there is one in taipei, otherwise it selects the one in Tainan. If none of them exists, nothing will be returned.
select sid
from supplier
where city = 'Taipei'
union all
select sid
from supplier
where city = 'Tainan'
and not exists (select 1 from supplier where city = 'taipei')