使用case函数合并2个SELECT查询

SELECT
    col1,
    col2,
    CASE
        WHEN condition1 THEN result1
        WHEN condition2 THEN result2
        ...
        ELSE result
    END AS col3
FROM
    table1

UNION

SELECT
    col1,
    col2,
    CASE
        WHEN condition3 THEN result3
        WHEN condition4 THEN result4
        ...
        ELSE result
    END AS col3
FROM
    table2;

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    ...
    ELSE result
END

SELECT
    name,
    position,
    CASE
        WHEN employee_id IS NOT NULL THEN '员工'
        ELSE '客户'
    END AS role
FROM
    employees

UNION

SELECT
    name,
    '',
    '客户' AS role
FROM
    customers;