有条件地联接到SQL Server中的不同表

Question

We have two tables with identical schemas in two different databases. We have a job that runs and uses data that joins to one or the other of those two tables. What we would ideally like to do (which I know isn't possible) is:

SELECT
    *
FROM
    OtherData d
    IF (condition)
        INNER JOIN database1.Table t ON ...
    ELSE
        INNER JOIN database2.Table t ON ...

We tried building out a view that unions the two datasets and returns the condition as part of the view, so we could select where the condition is true. If we just select against the view, the query plan shows that it only hits the desired database without going to the other. If we join to the view though, it goes to both.

CREATE VIEW View_Table AS
    SELECT
        1 AS Condition,
        OtherData...
    FROM...
        database1.Table
    UNION
    SELECT
        2 AS Condition,
        OtherData...
    FROM
        database2.Table
GO


-- Then use it as:
SELECT
    *
FROM
    OtherData d
    INNER JOIN View_Table vt ON vt.Condition = @Condition AND ...

So we are trying to figure out the best way (without just doing giant IF...ELSE... statements that duplicate large portions of our procs) to write this such that in execution we will only hit the desired database and not try to go to both when we know up front which to hit.

Answer 1

You can use the ISNULL function:

SELECT      d.*,
            ISNULL(T1.Field, T2.Field) Field
FROM        OtherData d
LEFT JOIN   database1.[Table] t1
        ON  d.Field = t1.Field AND (condition)
LEFT JOIN   database2.[Table] t2
        ON  d.Field = t2.Field AND (condition)

Or the CASE statement:

SELECT      d.*,    
            CASE    WHEN    (condition)
                    THEN    t1.Field
                    ELSE    t2.Field
            END Field
FROM        OtherData d
LEFT JOIN   database1.[Table] t1
        ON  d.Field = t1.Field
LEFT JOIN   database2.[Table] t2
        ON  d.Field = t2.Field

Answer 2

You could use a staging table to simplify your stored procedure.

Pseudosql...

truncate table stage.Table;

IF (condition)
    select * into stage.Table from database1.Table where ...
ELSE
    select * into stage.Table from database2.Table where ...

-- Then use it as:
SELECT
    *
FROM
    OtherData d
    INNER JOIN stage.Table ON ...

IMHO, I'd try to do any type of data flow with SSIS packages which would also make it easier to conditionally hit different databases (via Control Flow).

Answer 3

This is a case where dynamic SQL is probably your best bet.

And of course when telling people to use dynamic SQL, you must refer then to the page that describes the hazards of using it and how to protect your self when you must use it.

http://www.sommarskog.se/dynamic_sql.html

http://www.sommarskog.se/dyn-search.html