SQL从单行中选择2列作为列表

Question

I have a table with columns [From], [To] . I need to select these columns as a single list and I do it this way:

SELECT [From] FROM [TableX] WHERE (...)
UNION
SELECT [To] FROM [TableX] WHERE (...)

The problem is that I don't want to duplicate WHERE (...) condition as it's quite long. Is there a way to avoid this duplication?

Answer 1

Use a CTE to prevent duplicating your WHERE condition:

;WITH [CTE_Criteria] AS (
    SELECT [From], [To]
    FROM [TableX]
    WHERE (...)
)
SELECT [From] FROM [CTE_Criteria]
UNION
SELECT [To] FROM [CTE_Criteria];

Answer 2

Do the UNION in a derived table, and have one common WHERE clause.

select *
from
(
    SELECT [From] FROM [TableX] 
    UNION
    SELECT [To] FROM [TableX]
)
WHERE (...)