在这种情况下，如何从多个mysql表中选择数据？

好吧，我有一个有点奇怪的场景，我不能改变数据库结构，因为我没有这个权限。对于这个问题，我有三张桌子

表1-

  id(int) | name(varchar) | link_id(int) | link_to(tinyint)
..............................................................
    1       | value1        | 1            | 2
    2       | value2        | 3            | 2
    3       | value3        | 1            | 3
    4       | value4        | 2            | 3
    5       | value5        | 3            | 3

  id(int)  | data(varchar)
............................
    1       | some1       
    2       | some2       
    3       | some3       

    id(int) | data(varchar)
............................
    1       | another1       
    2       | another2       
    3       | another3       

表2-

  id(int) | name(varchar) | link_id(int) | link_to(tinyint)
..............................................................
    1       | value1        | 1            | 2
    2       | value2        | 3            | 2
    3       | value3        | 1            | 3
    4       | value4        | 2            | 3
    5       | value5        | 3            | 3

  id(int)  | data(varchar)
............................
    1       | some1       
    2       | some2       
    3       | some3       

    id(int) | data(varchar)
............................
    1       | another1       
    2       | another2       
    3       | another3       

表3-

  id(int) | name(varchar) | link_id(int) | link_to(tinyint)
..............................................................
    1       | value1        | 1            | 2
    2       | value2        | 3            | 2
    3       | value3        | 1            | 3
    4       | value4        | 2            | 3
    5       | value5        | 3            | 3

  id(int)  | data(varchar)
............................
    1       | some1       
    2       | some2       
    3       | some3       

    id(int) | data(varchar)
............................
    1       | another1       
    2       | another2       
    3       | another3       

现在让我解释一下：在

表1

中，

link\u id

是

表2

或

表3

的

id

字段，并且

link\u to

说明它是链接到表2还是表3

现在，我需要从所有3个表中获取组合数据，以便针对

表1

中的每个

名称

字段，从正确的表中获取

数据

字段，该表由该行中的link\u id和link\u to字段定义

---

有任何mysql查询可以实现这一点吗？

您可以尝试使用UNION

(
SELECT t1.id,t2.data
FROM table1 t1
    INNER JOIN table2 t2 ON t1.link_id=t2.id AND t1.link_to=2
) UNION (
SELECT t1.id,t3.data
FROM table1 t1
    INNER JOIN table3 t3 ON t1.link_id=t3.id AND t1.link_to=3
)

---

你可以试着和工会合作

(
SELECT t1.id,t2.data
FROM table1 t1
    INNER JOIN table2 t2 ON t1.link_id=t2.id AND t1.link_to=2
) UNION (
SELECT t1.id,t3.data
FROM table1 t1
    INNER JOIN table3 t3 ON t1.link_id=t3.id AND t1.link_to=3
)

---

我同意公认的答案。 还可以创建存储过程并允许搜索

# stored procedure
DELIMITER $$;CREATE PROCEDURE `search_by_value`(
    in_searchValue VARCHAR(255)
)
BEGIN
  DECLARE jointable INTEGER;

  SELECT link_to INTO jointable FROM table1 WHERE name = in_searchValue;
  IF (jointable = 2) THEN
    SELECT * FROM table1 t1 left join table2 t2 on t1.link_id = t2.id WHERE t1.name = in_searchValue;
  ELSEIF (jointable = 3) THEN
    SELECT * FROM table1 t1 left join table3 t3 on t1.link_id = t3.id WHERE t1.name = in_searchValue;
  END IF;
END $$
DELIMITER;
# call the sp
call search_by_value('value1');

---

我同意公认的答案。 还可以创建存储过程并允许搜索

# stored procedure
DELIMITER $$;CREATE PROCEDURE `search_by_value`(
    in_searchValue VARCHAR(255)
)
BEGIN
  DECLARE jointable INTEGER;

  SELECT link_to INTO jointable FROM table1 WHERE name = in_searchValue;
  IF (jointable = 2) THEN
    SELECT * FROM table1 t1 left join table2 t2 on t1.link_id = t2.id WHERE t1.name = in_searchValue;
  ELSEIF (jointable = 3) THEN
    SELECT * FROM table1 t1 left join table3 t3 on t1.link_id = t3.id WHERE t1.name = in_searchValue;
  END IF;
END $$
DELIMITER;
# call the sp
call search_by_value('value1');