SQL Server：从tableA或tableB OR Both中选择

Question

Is there a way to select from table A , table B or both tables in SQL Server? As much as posible, I don't want to use dynamic SQL. I've seen this in a similar question but it only selects on 1 table.

Declare @table = 1

SELECT *
FROM Table1
WHERE <stuff>
AND @Table = 1

UNION ALL

SELECT *
FROM Table2
WHERE <stuff>
AND @Table = 2

Answer 1

You can try this:

You have two solution about value @table.

You can consider @table = NULL about selection of both tables, or you can consider @table = value (for example 3 as sum of 1 and 2), so your query becomes:

If you choose @table = NULL:

Declare @table = NULL

SELECT *
FROM Table1
WHERE <stuff>
AND (@Table = 1 OR @table IS NULL)

UNION ALL

SELECT *
FROM Table2
WHERE <stuff>
AND (@Table = 2 OR @table IS NULL)

If you choose @Table = value (for example 3)

Declare @table = 3

SELECT *
FROM Table1
WHERE <stuff>
AND (@Table = 1 OR @table = 3)

UNION ALL

SELECT *
FROM Table2
WHERE <stuff>
AND (@Table = 2 OR @table = 3)

Answer 2

You can use the IF...ELSE statements to achieve this as shown below. I am assuming you want to use @table to control what the script should do.

Declare @table = 1

IF @table = 1 
BEGIN
    SELECT *
    FROM Table1  
    WHERE <stuff>
END

ELSE IF @table = 2
BEGIN
    SELECT *
    FROM Table2
    WHERE <stuff>
END

ELSE IF @table = 3 /* OR @table IN (1,2) --depends on how you want to handle this*/
BEGIN
    SELECT *
    FROM Table1
    WHERE <stuff>

    UNION ALL

    SELECT *
    FROM Table2
    WHERE <stuff>
END

The script will stop as soon as the condition is satisfied. For example IF @Table = 2, only the following sections of script will run:

ELSE IF @table = 2
BEGIN
    SELECT *
    FROM Table2
    WHERE <stuff>
END

Answer 3

Hope this helps:

SELECT maker, Product.model AS model_1, PC.model AS model_2, price
FROM Product INNER JOIN PC ON PC.model = Product.model
ORDER BY maker, PC.model;