如何仅用OR关键字获取两列distinct值及跨列distinct邮箱

先明确个关键点：OR是行级逻辑判断符，用来筛选满足任一条件的行，没法直接把两列的取值合并到一个结果集里。但如果硬要在查询里用上OR，可以这么写：

假设你的表叫loan_records，SQL语句如下：

SELECT DISTINCT email
FROM (
    SELECT 
        CASE 
            WHEN borrower IS NOT NULL THEN borrower 
            ELSE owner 
        END AS email
    FROM loan_records
    WHERE borrower IS NOT NULL OR owner IS NOT NULL
) AS combined_emails;

不过这写法有个局限：如果某一行里borrower和owner都有值，只会保留borrower的内容，会漏掉owner的取值。要是想完整拿到两列所有不重复的值，最靠谱的还是用UNION——要是你非得带上OR，可以改成下面这种（虽然OR在这里有点冗余，但满足要求）：

SELECT DISTINCT email
FROM (
    SELECT borrower AS email FROM loan_records
    UNION ALL
    SELECT owner AS email FROM loan_records
) AS all_emails
WHERE email IS NOT NULL OR email IS NOT NULL;

这个需求本质是拿两列所有邮箱的去重并集。同样，OR没法直接合并列，但可以结合子查询用上它，不过更推荐用UNION（自动去重，逻辑更清晰）：

带OR的写法 #

SELECT DISTINCT email
FROM (
    -- 把两列的所有值拆成单独行
    SELECT borrower AS email FROM loan_records
    UNION ALL
    SELECT owner AS email FROM loan_records
) AS all_emails
WHERE email IS NOT NULL OR email IS NOT NULL;

推荐写法（逻辑更清晰） #

SELECT DISTINCT borrower AS email FROM loan_records
UNION
SELECT DISTINCT owner AS email FROM loan_records;

效果截图示例 #

原始表数据

![原始表数据](表结构包含id、borrower、owner三列，示例数据：
1 | alice@test.com | bob@test.com
2 | bob@test.com | charlie@test.com
3 | dave@test.com | alice@test.com)

查询结果

![查询结果](结果仅含email列，去重后的值为：
alice@test.com
bob@test.com
charlie@test.com
dave@test.com)

内容的提问来源于stack exchange，提问作者Loopedfrog4