有没有最好的方法来连接多个表

Question

有人可以帮助 joining/merging table 如下所示。

如果部门列（depart_1、depart_2、depart_3）在一个 table 中，我知道该怎么做。但无法实现这种情况，因为它们处于不同的 tables.

我有将近 100 个字段，比如部门，所以也不太关心性能。

Answer 1

通过使用 JOIN 和 UNION

SELECT
  id, name gender, 1 as seq, depart_1 as department
FROM tab 1
UNION 
SELECT
  id, name gender, 2 as seq, depart_2 as department
FROM tab 1
UNION
SELECt
  tab1.id, tab2.name, tab1.gender, 3 as seq, tab2.depart_3 as department
FROM tab2 JOIN tab1 on tab2.id = tab1.id
UNION
SELECt
  tab1.id, tab2.name, tab1.gender, 4 as seq, tab2.depart_4 as department
FROM tab2 JOIN tab1 on tab2.id = tab1.id
UNION
SELECT
   tab1.id, tab3.name, tab1.gender, 5 as seq, tab3.depart_5 as department
FROM tab3 JOIN tab1 on tab3.id = tab1.id
UNION
SELECT
   tab1.id, tab3.name, tab1.gender, 6 as seq, tab3.depart_6 as department
FROM tab3 JOIN tab1 on tab3.id = tab1.id

每次查询读取一个部门信息。因此，您可以在每个查询中为 seq 列使用静态数字。

Answer 2

最好先进行所有联合，然后在最后执行较小的连接。

SELECT tab.id,
    tab.name,
    gen.gender,
    tab.seq,
    tab.deparment
FROM
    (SELECT id, name, 1 as seq, depart_1 as department FROM tab 1
    UNION 
    SELECT id, name , 2 as seq, depart_2 as department FROM tab 1
    UNION
    SELECT id, name, 3 as seq, depart_3 as department FROM tab 2
    UNION 
    SELECT id, name , 4 as seq, depart_4 as department FROM tab 2) tab LEFT JOIN 
    (SELECT DISTINCT id,gender FROM tab1 ) gen ON tab.id=gen.id

由于您是在配置单元中执行此操作，这将自动执行映射端连接，这将使您的查询更快。

有没有最好的方法来连接多个表

Is there a best way to join multiple tables

sql

hive

hiveql