如何仅在某些情况下应用UNION（MySQL）

我有以下-简化-查询：

SELECT id
FROM table_x
UNION ALL 
SELECT 0 AS id

但是，我只想在查询的第一部分返回至少1个结果时附加UNION部分，例如：

带有一个条目（id=1）的表x：

表x空：

table_x: empty
Current result: 0
Desired result: empty

---

有没有办法在查询中实现这一点？非常感谢您的光临。

只要您的桌子还在。你可以这样做

SELECT
IF ( a is NULL, NULL, CONCAT(a,",",b))
FROM(
SELECT
  (SELECT * From myTable WHERE id = 2) a
  ,(SELECT 0) b
  ) C

结果是空的

id=1的结果 会是

IF ( a is NULL, NULL, CONCAT(a,",",b))
1,0

---

非常感谢你的回答。我用以下方法解决了这个问题：

SELECT *
FROM (SELECT (SELECT COUNT(1) FROM table_x) AS counter,
        id 
    FROM table_x
    UNION ALL
    SELECT (SELECT COUNT(1) FROM table_x) AS counter,
        0 AS id) a
WHERE a.counter > 0

---

在您无法执行的查询中，这两个选项彼此都不知道。您可以将联合包装到子查询中，并在外部查询中过滤其结果。

SELECT *
FROM (SELECT (SELECT COUNT(1) FROM table_x) AS counter,
        id 
    FROM table_x
    UNION ALL
    SELECT (SELECT COUNT(1) FROM table_x) AS counter,
        0 AS id) a
WHERE a.counter > 0