根据字段的值联接表

Question

我在连接表时遇到了麻烦，有一个关于两个表的连接的条件。我有三个表，让我们假设它为table1，table2和table3，

table1
+---+
|id |
+---+

table2
+---------------+
|id | table1_id |
+---------------+

table3
+----------------------------+
| id | table1_id | table2_id |
+----------------------------+

现在，我的主表是“ table3”，我需要以这样的方式将主表与table1和table2联接：如果table3中存在table2_id的值，那么table2应该与table2_id联接，并且类似地，如果table1_id退出，则table1将与table1_id联接，例如：通过这种方式进入table3

+----------------------------+
| id | table1_id | table2_id |
|  1 |     1     |     0     | 
|  2 |     0     |     1     |
+----------------------------+

for the value of id = 1,
table1_id exists & table2_id is zero, so table1 should be joined,
for the id = 2,
table2_id exists & table1_id is zero, so table2 should be joined,
if there is a case that both exists then table2 should be given the priority i.e, the table2 will be joined, can anyone make me out of this prb pls..

Answer 1

您可以尝试建立可放入条件并根据条件执行查询的过程。

Answer 2

您可能可以使用LEFT JOIN进行操作，然后使用CASE语句对所需的结果列进行排序。

例如，您可以执行以下操作。 请注意，您需要为要返回的每个字段重复CASE语句。

SELECT table3.id, CASE table2.id IS NULL THEN table1.field ELSE table2.field END AS field
FROM table3
LEFT OUTER JOIN table2 ON table3.table2_id = table2.id
LEFT OUTER JOIN table1 ON table3.table1_id = table1.id