mysql选择多行等于第2行

Question

我有两张桌子：

TableA          TableB
ID_A            ID_B
P1              Text
P2
P3
P4

-

TableB
ID_B    Text
1       A
2       B
3       C
4       D
5       E
6       F
7       G

-

TableA
ID_A    P1  P2  P3  P4
1       7   5   6   6

感谢尼克帮助我USER Nick

SELECT a.*, b.*
FROM (SELECT ID_A, P1 AS P FROM TableA
  UNION ALL
  SELECT ID_A, P2 FROM TableA
  UNION ALL
  SELECT ID_A, P3 FROM TableA
  UNION ALL
  SELECT ID_A, P4 FROM TableA) p
JOIN TableA a ON a.ID_A = p.ID_A
JOIN TableB b ON b.ID_B = p.P
ORDER BY a.ID_A, b.ID_B

我明白了：

ID_A    P1  P2  P3  P4    ID_B Text
1       7   5   6   6      5    E
1       7   5   6   6      6    F
1       7   5   6   6      6    F
1       7   5   6   6      7    G

我怎么能得到这个？

ID_A    P1  P2  P3  P4    ID_B Text
1       7   5   6   6      7    G
1       7   5   6   6      5    E
1       7   5   6   6      6    F
1       7   5   6   6      6    F

1°行P1 = ID_B

2°行P2 = ID_B

3°行P3 = ID_B

4°行P4 = ID_B

Demo on db-fiddle

Answer 1

发生这种情况是因为P值的派生表中没有顺序。为了能够对此进行排序，我们可以添加另一个表示P数字的字段（如P1，P2等）并按顺序排列而不是ID_B：

SELECT a.*, b.*
FROM (SELECT 1 AS PNUM, ID_A, P1 AS P FROM TableA
      UNION ALL
      SELECT 2, ID_A, P2 FROM TableA
      UNION ALL
      SELECT 3, ID_A, P3 FROM TableA
      UNION ALL
      SELECT 4, ID_A, P4 FROM TableA) p
JOIN TableA a ON a.ID_A = p.ID_A
JOIN TableB b ON b.ID_B = p.P
ORDER BY a.ID_A, p.PNUM

输出：

ID_A    P1  P2  P3  P4  ID_B    Text
1       7   5   6   6   7       G
1       7   5   6   6   5       E
1       7   5   6   6   6       F
1       7   5   6   6   6       F

Demo on dbfiddle

Answer 2

您可以使用CASE，WHEN作为自定义排序顺序。这将在期望的排序中显示结果

 SELECT a.*, b.*
 FROM (SELECT ID_A, P1 AS P FROM TableA
  UNION ALL
  SELECT ID_A, P2 FROM TableA
  UNION ALL
  SELECT ID_A, P3 FROM TableA
  UNION ALL
  SELECT ID_A, P4 FROM TableA) p
 JOIN TableA a ON a.ID_A = p.ID_A
 JOIN TableB b ON b.ID_B = p.P
 ORDER BY CASE b.ID_B
     WHEN 7 THEN 4
     WHEN 5 THEN 3
     ELSE 2
 END DESC

Answer 3

SELECT * FROM TableA as a1
left join TableB as b1 on ID_B = p1
union ALL
SELECT * FROM TableA
left join TableB on ID_B = p2
union ALL
SELECT * FROM TableA
left join TableB on ID_B = P3
union ALL
SELECT * FROM TableA
left join TableB on ID_B = p4