我的mysql表看起来像这样:
word1 word2 count
a c 1
a d 2
a e 3
a f 4
b c 5
b d 6
b g 7
b h 8
“a”和“b”是用户输入 - 从表中选择*,其中word1 ='a'或word1 ='b' - 得到~10000行
我需要一个查询来获取: word1是输入“a”
的列word1_是输入“b”
的列word2和word2_是同一列,因此可以忽略其中一个
我需要结合上表中的表格。 例如这个查询:
select
t1.word1, t1.word2, t1.count,
t2.word1 as word1_, t2.word2 as word2_, t2.count as count_
from table t1
join table t2 on t1.word2 = t2.word2
where t1.word1 = 'a' and t2.word1 = 'b'
产生
word1 word2 count word1_ word2_ count_
a c 1 b c 5
a d 2 b d 6
我需要得到count = 0,其中找不到word2。
word1 word2 count word1_ word2_ count_
a c 1 b c 5
a d 2 b d 6
a e 3 b e 0
a f 4 b f 0
a g 0 b g 7
a h 0 b h 8
P.S。该表在word1
P.P.S。提供的答案确实有效,但完成查询需要20秒。我需要自己编程以获得更好的性能。
答案 0 :(得分:5)
你需要一个FULL OUTER JOIN ......这在mysql中是不存在的。
你可以这样做。
select
t1.word1, t1.word2, t1.count,
coalesce(t2.word1, 'b') as word1_, t1.word2 as word2_, coalesce(t2.count, 0) as count_
from table1 t1
left join table1 t2 on t1.word2 = t2.word2 and t2.word1 = 'b'
where t1.word1 = 'a'
union
select
coalesce(t2.word1, 'a'), t1.word2 , coalesce(t2.count, 0),
t1.word1 as word1_, t1.word2 as word2_, t1.count
from table1 t1
left join table1 t2 on t1.word2 = t2.word2 and t2.word1='a'
where t1.word1 = 'b'
请参阅SqlFiddle
答案 1 :(得分:1)
你真的不需要任何UNION或OUTER JOIN
SELECT 'a' word1
, b.word2
, max(CASE word1 WHEN 'a' THEN count ELSE 0 END) count
, 'b' _word1
, b.word2 _word2
, max(CASE word1 WHEN 'b' THEN count ELSE 0 END) _count
FROM words a
INNER JOIN (SELECT DISTINCT word2
FROM words
WHERE word1 IN ('a', 'b')) b ON a.word2 = b.word2
GROUP BY b.word2
ORDER BY b.word2
演示:SQLFiddle
在演示中,我添加了一行,其中word1既不是' a'或者' b',如果你想要word2的每个值而不管word1的值只是去掉子查询的WHERE条件