Case statement to determine if I should union

问题

I currently want to do some sort of conditional union. Given the following example:

SELECT  age, name
FROM    users
UNION
SELECT  25 AS age, 'Betty' AS name

Say I wanted to only union the second statement if the count of 'users' was >=2 , otherwise do not union the two.

In summary I want to append a table with a row if the table only has 2 or more values.

回答1:

You could use ~~an ugly hack~~ something like this, but I think Tim's answer is better:

SELECT  age, name
FROM    users
UNION ALL

SELECT 25 AS age, 'Betty' AS name
WHERE (SELECT COUNT(*) FROM users) > 1

回答2:

If it's in a stored-procedure you could use If...Else:

IF (SELECT COUNT(*) FROM users) < 2
 BEGIN
  SELECT  age, name
  FROM    users
  END
ELSE
  SELECT  age, name
  FROM    users
  UNION ALL
  SELECT  25 AS age, 'Betty' AS name

Otherwise you could try something like this:

SELECT  age, name
  FROM    users
  UNION ALL
  SELECT  TOP 1 25 AS age, 'Betty' AS name
  FROM users
  WHERE (SELECT COUNT(*) FROM users) >= 2

Note that i've used UNION ALL since it doesn't seem that you want to eliminate duplicates.

Played around here: http://sqlfiddle.com/#!6/a7540/2323/0

Edit: Instead of my second approach i prefer Zohar's. So if you can use If....Else prefer that otherwise WHERE (SELECT COUNT(*) FROM users) > 1 without a table.

回答3:

Something like the following should work:

SELECT  age, name
FROM    users

UNION ALL

SELECT age, name
FROM (SELECT  25 AS age, 'Betty' AS name) x
CROSS APPLY (SELECT COUNT(*) FROM users) y(cnt)
WHERE y.cnt >= 2

Second part of UNION ALL will be NULL in case users table has less than 2 records.

回答4:

SELECT age
     , name
FROM   users

UNION

SELECT 25 As age
     , 'Betty' As name
WHERE  EXISTS (
         SELECT Count(*)
         FROM   users
         HAVING Count(*) >= 2
       )
;

来源：https://stackoverflow.com/questions/30326880/case-statement-to-determine-if-i-should-union

标签

sql

sql-server

sql-server-2012

case

union