在 SQL 查询中使用另一个 CASE 表达式中的列值

Question

I am trying to user one column obtained from the CASE expression to SET another Column Value.

EG I have a table user with multiple column. First, I need to set the status of a project based on the values of some columns in the table. I am able to make this part work. The second part is, based on the status value, I need to get the user name(s) from multiple columns. When I do that, am getting an error

Invalid Column 'Status'

A sample query follows:

  SELECT
  STATUS = CASE WHEN ISNULL(USER1,'') = '1' THEN 'Approve'
           WHEN ISNULL(USER2,'') = '2' THEN 'Reject'
           WHEN ISNULL(USER3,'') = '3' THEN 'Pending Decision'
           ELSE 'error' END,
 USERNAME = CASE WHEN ISNULL(STATUS,'') = 'Approve' THEN (select username1 from <table> where userid = <userid>)
           WHEN ISNULL(STATUS,'') = 'Reject' THEN  (select username2 from <table> where userid = <userid>)
           WHEN ISNULL(STATUS,'') = 'Pending Decision' THEN  (select username3 from <table> where userid = <userid>)
           ELSE 'error' END

           from <table> where userid = <userid>

Answer 1

This works, because @status will be processed before username

And you have to use user defined variables, you can't use columnnames for that

  SELECT
  @status := (CASE WHEN COALESCE(USER1,'') = '1' THEN 'Approve'
           WHEN COALESCE(USER2,'') = '2' THEN 'Reject'
           WHEN COALESCE(USER3,'') = '3' THEN 'Pending Decision'
           ELSE 'error' END)  `STATUS` 
           ,
  CASE WHEN COALESCE(@status,'') = 'Approve' THEN (select username1 from table1 where userid = 20)
          WHEN COALESCE(@status,'') = 'Reject' THEN  (select username2 from table1 where userid = 20)
           WHEN COALESCE(@status,'') = 'Pending Decision' THEN  (select username3 from table1 where userid = 20)
           ELSE 'error' END USERNAME 

           from table1 where userid = 20

Answer 2

You can use a "table expression" to pre-compute a column. For example:

select
  *,
  -- use the pre-computed STATUS column now
from (
  SELECT *,
    <CASE-expression> as status
  from <table> where userid = <userid>
) x

Answer 3

You can't reuse an expression defined in the select clause in the same clause.

I don't really see why you need the value returned by the first expression to compute the second value, since the logic is basically the same.

You can just do:

select
    case 
        when user1 = 1 then then 'Approve'
        when user2 = 2 then 'Reject'
        when user3 = 3 then 'Pending Decision'
        else 'error' 
    end status,
    case 
        when user1 = 1 then (select username1 from <table> where userid = <userid>)
        when user2 = 2 then (select username2 from <table> where userid = <userid>)
        when user3 = 3 then (select username3 from <table> where userid = <userid>)
    end username
from <table> where userid = <userid>

Notes

• isnull() is unnecessary, since the alternative value is not trapped anywhere in the case statement

• this treats user1 as a number, because it looks like it - so I unquoted the test values in the case statement. If it's actually a string, you can revert this and add the single quotes.

• I am quite suspicious about the logic of the subqueries; there might be ways to simplify that, if you were to provide more details on what they are intented to do, along with sample data and desired results.