预测输出中的行数

Question

我有下表：

col_a	col_b
0	0
1	0
0	0
1	1
1	1
2	0

在 col_a 和 col_b 中加入表后的行数是多少 - 左、右和内？ 我想了解 SQL 连接的工作原理。

Answer 1

Sql 中不止有连接

create table test (col_a int, col_b int);
insert into test values (1,1),(2,3),(3,2),(4,0);

select * from test order by 1;

col_a | col_b ----: | ----: 1 | 1 2 | 3 3 | 2 4 | 0

--
-- [inner] join: only when a = b
--
select a.col_a, b.col_b
from test a
join test b 
  on b.col_b = a.col_a

col_a | col_b ----: | ----: 1 | 1 2 | 2 3 | 3

--
-- left [outer] join: all of left side a
--
select a.col_a, b.col_b
from test a
left join test b 
  on b.col_b = a.col_a

col_a | col_b ----: | ----: 1 | 1 2 | 2 3 | 3 4 | 空

--
-- right [outer] join, all of right side b
--
select a.col_a, b.col_b
from test a
right join test b 
  on b.col_b = a.col_a

col_a | col_b ----: | ----: 空 | 0 1 | 1 2 | 2 3 | 3

--
-- full [outer] join: all of left a and all of right b
--
select a.col_a, b.col_b
from test a
full join test b 
  on b.col_b = a.col_a

col_a | col_b ----: | ----: 空 | 0 1 | 1 2 | 2 3 | 3 4 | 空

--
-- natural join: an inner join, but automatic on the same column names
--
select a.col_a, b.col_b
from test a
natural join test b 

col_a | col_b ----: | ----: 1 | 1 2 | 3 3 | 2 4 | 0

--
--  some join with a "using" instead of "on" 
--
select a.col_a, b.col_b
from test a
inner join test b using (col_a)

col_a | col_b ----: | ----: 1 | 1 2 | 3 3 | 2 4 | 0

--
-- cross join: a * b (cartesian product)
--
select a.col_a, b.col_b
from test a
cross join test b 

col_a | col_b ----: | ----: 1 | 1 1 | 3 1 | 2 1 | 0 2 | 1 2 | 3 2 | 2 2 | 0 3 | 1 3 | 3 3 | 2 3 | 0 4 | 1 4 | 3 4 | 2 4 | 0

--
-- lateral join: 
--
select a.col_a, lat_b.col_b
from test a
cross join lateral (
  select b.col_b, count(*) as total
  from test b 
  where b.col_b = a.col_a
  group by b.col_b
) lat_b

col_a | col_b ----: | ----: 1 | 1 2 | 2 3 | 3

--
-- correlated sub-query
--
select 
  a.col_a, 
  (select max(col_b) from test b where a.col_a = b.col_b) as col_b
from test a

col_a | col_b ----: | ----: 1 | 1 2 | 2 3 | 3 4 | 空

--
-- where b in a
--
select a.col_a
from test a
where col_a in (select distinct col_b from test b)

| col_a | | ----: | | 1 | | 2 | | 3 |

--
-- where tupple in opposite tupple
--
select col_a, col_b
from test a
where (col_a, col_b) in (select col_b, col_a from test b)

col_a | col_b ----: | ----: 1 | 1 2 | 3 3 | 2

--
-- where b exists in a
--
select a.col_a
from test a
where exists (
  select 1
  from test b 
  where b.col_b = a.col_a
)

| col_a | | ----: | | 1 | | 2 | | 3 |

--
-- union all: stack a and b
--
select col_a as col_ab from test a
union all
select col_b from test b

| col_ab | | -----: | | 1 | | 2 | | 3 | | 4 | | 1 | | 3 | | 2 | | 0 |

--
-- union: stack a and b, but discard duplicates
--
select col_a as col_ab from test a
union
select col_b from test b

| col_ab | | -----: | | 2 | | 4 | | 1 | | 3 | | 0 |

--
-- intersect: only when the value is in both
--
select col_a as col_ab from test a
intersect
select col_b from test b

| col_ab | | -----: | | 3 | | 2 | | 1 |

--
-- except: a minus b
--
select col_a as col_ab from test a
except
select col_b from test b

| col_ab | | -----: | | 4 |

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