sql左联接获取与左联接匹配的记录

Question

如果指定了右表列值，如何仅基于右表中的列获取具有左连接的匹配记录。

例如。

SELECT A.* FROM A LEFT JOIN B ON A.COLUMN_1 = B.COLUMN_1 and B.COLUMN_1 IN (1,2)

-- in above case i want to get only matching records with B.COLUMN_1 is 1 or 2.

如果未指定B.COLUMN_1，则需要从左表获取所有记录。

Answer 1

SELECT 
    A.* 
FROM 
    A 
LEFT JOIN (
    SELECT 
        B.* 
    FROM 
        B 
    WHERE 
        B.COLUMN_1 IN (1,2)
) BB ON A.COLUMN_1 = BB.COLUMN_1

Answer 2

您可以使用两种形式的UNION来获取所有数据，例如

SELECT A.* 
FROM A 
LEFT JOIN B 
ON A.COLUMN_1 = B.COLUMN_1 
and B.COLUMN_1 IN (1,2)

UNION

SELECT A.* 
FROM A 
JOIN B ON A.COLUMN_1 = B.COLUMN_1 
and B.COLUMN_1 IN (1,2)