Question

是否可以不使用union运算符组合sql中的查询结果（与union / union all相同）

Answer 1

假设您有两个表格UNION

CREATE TABLE Table1
(`id` int, `name` varchar(32));
CREATE TABLE Table2
(`id` int, `name` varchar(32));

样本数据

表1：

| ID |  NAME |
|----|-------|
|  1 | name1 |
|  2 | name2 |
|  3 | name3 |

表2：

| ID |  NAME  |
|----|--------|
| 11 | name11 |
| 22 | name22 |
| 33 | name33 |
|  1 |  name1 |

模仿UNION ALL

SELECT COALESCE(t1.id, t2.id) id,
       COALESCE(t1.name, t2.name) name
  FROM 
(
  SELECT TABLE_NAME <> 'table1' n
    FROM INFORMATION_SCHEMA.TABLES
   WHERE TABLE_SCHEMA = SCHEMA()
     AND TABLE_NAME IN('table1', 'table2')
) t LEFT JOIN table1 t1 
    ON t.n = 0 LEFT JOIN table2 t2
    ON t.n = 1

输出：

| ID |   NAME |
|----|--------|
|  1 |  name1 |
|  2 |  name2 |
|  3 |  name3 |
| 11 | name11 |
| 22 | name22 |
| 33 | name33 |
|  1 |  name1 |

要模仿UNION，您只需添加DISTINCT

即可

SELECT DISTINCT COALESCE(t1.id, t2.id) id,
       COALESCE(t1.name, t2.name) name
  FROM 
(
  SELECT TABLE_NAME <> 'table1' n
    FROM INFORMATION_SCHEMA.TABLES
   WHERE TABLE_SCHEMA = SCHEMA()
     AND TABLE_NAME IN('table1', 'table2')
) t LEFT JOIN table1 t1 
    ON t.n = 0 LEFT JOIN table2 t2
    ON t.n = 1

输出：

| ID |   NAME |
|----|--------|
|  1 |  name1 |
|  2 |  name2 |
|  3 |  name3 |
| 11 | name11 |
| 22 | name22 |
| 33 | name33 |

这是 SQLFiddle 演示

Answer 2

我们尝试并获得了简单的查询，但我不知道这有多高效。

CREATE TABLE tableA
  ( col1 INTEGER , col2 INTEGER
  );
CREATE TABLE tableB
  ( col1 INTEGER , col2 INTEGER
  )

// insert statements
INSERT INTO tableA
  (COL1, COL2
  ) VALUES
  (12, 13
  );
INSERT INTO tableA
  (COL1, COL2
  ) VALUES
  (14, 15
  );
INSERT INTO tableB
  (COL1, COL2
  ) VALUES
  (12, 13
  );
INSERT INTO tableB
  (COL1, COL2
  ) VALUES
  (14, 15
  );

// for union all
SELECT (
  CASE
    WHEN b.col1 IS NULL
    THEN a.col1
    ELSE b.col1
  END)
FROM
  (SELECT COL1 FROM TableB
  )b
FULL OUTER JOIN
  (SELECT COL1 FROM TableA
  ) a
ON 1=2

// for union
SELECT (
  CASE
    WHEN b.col1 IS NULL
    THEN a.col1
    WHEN b.col1=a.col1
    THEN b.col1
    ELSE b.col1
  END)
FROM
  (SELECT COL1 FROM TableB
  )b
FULL OUTER JOIN
  (SELECT COL1 FROM TableA
  ) a
ON a.col1=b.col1

在不使用集合运算符的情况下组合选择查询的结果（UNION）

2 个答案: