sql - How can I check one table for an entry but use default value if it doesn't exist?

Question

I have 2 tables. TableA has columns OldLogin and NewLogin. TableB has ID, IsManager, and CanChange.

I'm writing a stored procedure with an input parameter login. It needs ultimately return all columns in TableB where login = ID but it first needs to check TableA to see if it needs to use NewLogin.

So, can someone help me write a query that says "show me all columns in TableB where ID = (login OR NewLogin)"?

Thanks!

EDIT: Added code:

DECLARE @login varchar(30) = 'test'

SELECT ISNULL(NewLogin, @login) AS NewLogin, TableB .IsManager, TableB.CanChange
FROM TableA
LEFT JOIN TableB ON TableB.ID = ISNULL(NewLogin, @login)
WHERE OldLogin = @login

THe above code works only if @login exists in TableA. I get no results if it doesn't exist in that table.

Answer 1

如果 a 为空，则使用 IFNULL(a,b) 返回 b。

编辑：我假设 mysql，不确定它是否适用于其他语言。您也可以使用 COALESCE，它接受任意数量的参数并返回第一个非空参数。

DECLARE @login varchar(30) = 'test'

SELECT IFNULL(TableA.NewLogin, @login) AS NewLogin, TableB.IsManager, TableB.CanChange
FROM TableA
LEFT JOIN TableB ON 
  TableB.ID = IFNULL(TableA.NewLogin, @login)
WHERE OldLogin = @login

Answer 2

DECLARE @login varchar(30) = 'test'
SELECT DISTINCT TableB.*
FROM TableA
  INNER JOIN TableB ON TableB.ID = NewLogin OR TableB.ID = @login

Answer 3

if I'm understanding that correctly...

SELECT TableB.*
FROM TableB
LEFT JOIN TableA ON TableB.id IN (TableA.OldLogin, TableB.NewLogin)

?