Postgresql加入同一个表

Question

我有桌子的字段：

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

对于结果，我需要每行line_id有一行的表，并且point_address值line_id都在不同的行中：

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

我试图创建自联接，但我得到的是每个line_id的4个结果，包含point_address字段的所有组合。获得单一结果的最佳方法是什么？

Answer 1

假设每个不同的line_id只有两个地址，你可以通过在结果表中强制命令point_address来解决：

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');

选择产量

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

当您尝试将行创建为列时，关系代数不会很好，反之亦然。