前段時間在實際工作中，使用者的一條sql引發了我一些思考。寫一篇簡單的博文來記錄下。實際表的列名等已替換。
select
*from
db1.table1 as t1
left
join db2.table2 as t2 on
t1.col1 = t2.col2
where
t1.time
>=
'2020-01-01 00:00:00'
and t1.
time
<
'2020-01-31 00:00:00'
and t1.col2 in
('5k'
)
code: 48. db::exception: received from localhost:10115. db::exception:method createcolumn() is not implemented for data type set.
select
*from
db1.table1 as t1
left
join db2.table2 as t2 on
t1.col1 = t2.col2
where
t1.time
>=
'2020-01-01 00:00:00'
and t1.
time
<
'2020-01-31 00:00:00'
select
*from
db1.table1 as t1
left
join db2.table2 as t2 on
t1.col1 = t2.col2
where
t1.col2 in
('5k'
)
通過分析，這條sql就是一條join語句，並且過濾條件全部是一張表的列，那麼我們是否可以先利用過濾字段查出一張自表之後，再去join呢？帶著這樣的思路，我們修改sql如下：
select
*from
(select
*from db1.table1 where col2 in
('5k'))
as t1
left
join db2.table2 as t2 on
t1.col1 = t2.col2
where
t1.time
>=
'2020-01-01 00:00:00'
and t1.
time
<
'2020-01-31 00:00:00'
select
*from
(select col1,col2,
time
from db1.table1 where col2 in
('5k'))
as t1
left
join db2.table2 as t2 on
t1.col1 = t2.col2
where
t1.time
>=
'2020-01-01 00:00:00'
and t1.
time
<
'2020-01-31 00:00:00'
select
*from
(select col1,col2,
time
from db1.table1)
as t1
left
join db2.table2 as t2 on
t1.col1 = t2.col2
where
t1.time
>=
'2020-01-01 00:00:00'
and t1.
time
<
'2020-01-31 00:00:00'
and t1.col2 in
('5k'
)
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