[英]postgresql - sql - count of `true` values
myCol
------
true
true
true
false
false
null
In the above table, if I do:在上表中,如果我这样做:
select count(*), count(myCol);
I get 6, 5
我得到
6, 5
I get 5
as it doesn't count the null entry.我得到
5
,因为它不计算空条目。
How do I also count the number of true values (3 in the example)?我如何计算真值的数量(示例中为 3)?
(This is a simplification and I'm actually using a much more complicated expression within the count function) (这是一个简化,我实际上在计数函数中使用了一个更复杂的表达式)
Edit summary: I also want to include a plain count(*) in the query, so can't use a where clause编辑摘要:我还想在查询中包含一个普通的 count(*),所以不能使用 where 子句
SELECT COALESCE(sum(CASE WHEN myCol THEN 1 ELSE 0 END),0) FROM <table name>
or, as you found out for yourself:或者,正如您自己发现的那样:
SELECT count(CASE WHEN myCol THEN 1 END) FROM <table name>
Since PostgreSQL 9.4 there's the FILTER
clause , which allows for a very concise query to count the true values:由于 PostgreSQL 9.4 有
FILTER
子句,它允许非常简洁的查询来计算真实值:
select count(*) filter (where myCol)
from tbl;
The above query is a bad example in that a simple WHERE clause would suffice, and is for demonstrating the syntax only.上面的查询是一个不好的例子,因为一个简单的 WHERE 子句就足够了,并且仅用于演示语法。 Where the FILTER clause shines is that it is easy to combine with other aggregates:
FILTER 子句的亮点在于它很容易与其他聚合结合使用:
select count(*), -- all
count(myCol), -- non null
count(*) filter (where myCol) -- true
from tbl;
The clause is especially handy for aggregates on a column that uses another column as the predicate, while allowing to fetch differently filtered aggregates in a single query:该子句对于使用另一列作为谓词的列上的聚合特别方便,同时允许在单个查询中获取不同过滤的聚合:
select count(*),
sum(otherCol) filter (where myCol)
from tbl;
Cast the Boolean to an integer and sum.将布尔值转换为整数并求和。
SELECT count(*),sum(myCol::int);
You get 6,3
.你得到
6,3
。
probably, the best approach is to use nullif function.可能最好的方法是使用 nullif 函数。
in general一般来说
select
count(nullif(myCol = false, true)), -- count true values
count(nullif(myCol = true, true)), -- count false values
count(myCol);
or in short或简而言之
select
count(nullif(myCol, true)), -- count false values
count(nullif(myCol, false)), -- count true values
count(myCol);
http://www.postgresql.org/docs/9.0/static/functions-conditional.html http://www.postgresql.org/docs/9.0/static/functions-conditional.html
The shortest and laziest (without casting) solution would be to use the formula:最短和最懒惰(没有铸造)的解决方案是使用以下公式:
SELECT COUNT(myCol OR NULL) FROM myTable;
Try it yourself:自己试试:
SELECT COUNT(x < 7 OR NULL)
FROM GENERATE_SERIES(0,10) t(x);
gives the same result than给出相同的结果
SELECT SUM(CASE WHEN x < 7 THEN 1 ELSE 0 END)
FROM GENERATE_SERIES(0,10) t(x);
select f1,
CASE WHEN f1 = 't' THEN COUNT(*)
WHEN f1 = 'f' THEN COUNT(*)
END AS counts,
(SELECT COUNT(*) FROM mytable) AS total_counts
from mytable
group by f1
Or Maybe this或者也许这个
SELECT SUM(CASE WHEN f1 = 't' THEN 1 END) AS t,
SUM(CASE WHEN f1 = 'f' THEN 1 END) AS f,
SUM(CASE WHEN f1 NOT IN ('t','f') OR f1 IS NULL THEN 1 END) AS others,
SUM(CASE WHEN f1 IS NOT NULL OR f1 IS NULL THEN 1 ELSE 0 END) AS total_count
FROM mytable;
Simply convert boolean field to integer and do a sum.只需将布尔字段转换为整数并进行求和。 This will work on postgresql:
这将适用于 postgresql:
select sum(myCol::int) from <table name>
Hope that helps!希望有帮助!
In MySQL, you can do this as well:在 MySQL 中,您也可以这样做:
SELECT count(*) AS total
, sum(myCol) AS countTrue --yes, you can add TRUEs as TRUE=1 and FALSE=0 !!
FROM yourTable
;
I think that in Postgres, this works:我认为在 Postgres 中,这是可行的:
SELECT count(*) AS total
, sum(myCol::int) AS countTrue --convert Boolean to Integer
FROM yourTable
;
or better (to avoid:: and use standard SQL syntax):或更好(避免::并使用标准SQL语法):
SELECT count(*) AS total
, sum(CAST(myCol AS int)) AS countTrue --convert Boolean to Integer
FROM yourTable
;
SELECT count(*) -- or count(myCol)
FROM <table name> -- replace <table name> with your table
WHERE myCol = true;
Here's a way with Windowing Function:这是窗口功能的一种方法:
SELECT DISTINCT *, count(*) over(partition by myCol)
FROM <table name>;
-- Outputs:
-- --------------
-- myCol | count
-- ------+-------
-- f | 2
-- t | 3
-- | 1
TL;DR: take the solution you like. TL;DR:采用您喜欢的解决方案。 There's no significant difference.
没有显着差异。
before(){
psql <<-SQL
create table bench (
id serial
, thebool boolean
);
insert into bench (thebool)
select (random() > 0.5)
from generate_series(1, 1e6) g;
analyze bench;
SQL
}
after(){
psql -c 'drop table bench'
}
test(){
echo $(tput bold)$1$(tput sgr0)
psql -c "explain analyze select $1 from bench" | tail -4 | head -2
}
Made on a 1.4GHz i5 MacBookPro, psql and pg 12.4 (pg in a linux docker container):在 1.4GHz i5 MacBookPro、psql 和 pg 12.4 上制作(pg 在 linux docker 容器中):
before
test 'count(*) filter (where thebool)'
# Planning Time: 0.138 ms
# Execution Time: 4424.042 ms
test 'count(case when thebool then 1 end)'
# Planning Time: 0.156 ms
# Execution Time: 4638.861 ms
test 'count(nullif(thebool, false))'
# Planning Time: 0.201 ms
# Execution Time: 5267.631 ms
test 'count(thebool or null)'
# Planning Time: 0.202 ms
# Execution Time: 4672.700 ms
test 'sum(thebool::integer)'
# Planning Time: 0.155 ms
# Execution Time: 4602.406 ms
test 'coalesce(sum(case when thebool THEN 1 ELSE 0 END), 0)'
# Planning Time: 0.167 ms
# Execution Time: 4416.503 ms
after
select count(myCol)
from mytable
group by myCol
;
will group the 3 possible states of bool (false, true, 0) in three rows especially handy when grouping together with another column like day将 bool (false, true, 0) 的 3 种可能状态分组在三行中,当与另一列(如 day)分组在一起时特别方便
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