Question

我有带有字段的表：

line_id, point_address:
1 | Address1
1 | Address2
2 | Address1
2 | Address3
3 | Address2
3 | Address3

为得到结果，我需要每个line_id都具有一行的表，并且line_id的两个point_address值都在单独的行中：

line_id,point_address_1,point_address_2:
1 | Address1 | Address2
2 | Address1 | Address3
3 | Address2 | Address3

我试图创建自我联接，但是我得到的是每个line_id具有point_address字段的所有组合的4个结果。取得单一结果的最佳方法是什么

Answer 1

假设每个line_id上只有两个地址，则可以通过在结果表中强制point_address上的顺序来解决此问题：

SELECT
    l1.line_id, l1.point_address AS point_address1, 
        l2.point_address AS point_address2
    FROM line_address l1 JOIN line_Address l2 ON l1.line_id = l2.line_id
    WHERE l1.point_address < l2.point_address;

line_id     point_address1  point_address2
----------  --------------  --------------
1           Address1        Address2      
2           Address1        Address3      
3           Address2        Address3

请注意，这仅在上述假设下有效，因此，如果我们添加

INSERT INTO line_address (line_id, point_address) VALUES (3, 'Address4');

精选产量

line_id     point_address1  point_address2
----------  --------------  --------------
1           Address1        Address2      
2           Address1        Address3      
3           Address2        Address3      
3           Address2        Address4      
3           Address3        Address4

当您尝试将行划分为列时，关系代数并不那么友好。

PostgreSQL加入同一张表

1 个答案: