我想运行一个查询,对于给定的字段将计算特定字符的实例。
例如,如果我有一个名为“Friends”的表,其中的“Names”字段包含以下行:Edward、James、Mike。我会得到输出:
A 2、D 2、E 3、I 1 等等……
一般的答案是,您需要将每个名称拆分成一个组成字符表,然后计算它们。
你没有提到你正在使用哪个 RDBMS。答案因数据库引擎而异。
例如,这将适用于 SQL 2005+:
DECLARE @friends TABLE (NAMES VARCHAR(30))
DECLARE @maxLen INT
INSERT @friends (NAMES)
SELECT 'Edward'
UNION SELECT 'James'
UNION SELECT 'Mike'
SELECT @maxLen = MAX(LEN(NAMES)) FROM @friends
;WITH numsCte
AS
(
--dynamic numbers table. If you have a numbers table in your database
--use that instead, as it will be more efficient.
SELECT 1 AS n
UNION ALL
SELECT n+1 FROM numsCte
WHERE n < @maxLen
)
,charCTE
AS
(
--split the string into a dataset
SELECT *
FROM numsCte AS nm
CROSS APPLY (SELECT NAMES, SUBSTRING(NAMES, n, 1) AS splitChar
FROM @friends
) AS st
WHERE splitChar > ''
)
SELECT UPPER(splitChar) AS letter
,COUNT(1) AS cnt
FROM charCTE
GROUP BY splitChar
ORDER BY splitChar
但几乎可以肯定不会在任何其他数据库引擎上工作。
一种方法是使用临时表,并在 WHILE 循环中填充它:
declare @letters table (letter varchar(1))
declare @pos int
set @pos = 1
while 1=1
begin
insert into @letters
select substring(name,@pos,1)
from @names
where len(name) >= @pos
if @@rowcount = 0
break
set @pos = @pos + 1
end
select letter, count(*)
from @letters
group by letter
另一种方法是在临时表中创建一个有效字符位置的列表,或者如本例所示,使用递归公用表表达式 (CTE):
declare @maxLen int
select @maxLen = max(len(name)) from @names
;WITH CharPositions (i) AS (
select 1
union all
select i+1
from CharPositions
where i < @maxLen
)
select substring(n.name,cp.i,1), count(*)
from @names n
inner join CharPositions cp on cp.i <= len(n.name)
group by substring(n.name,cp.i,1)
我已经针对这个数据集测试了代码示例:
declare @names table (name varchar(max))
insert into @names values ('abc')
insert into @names values ('def')
insert into @names values ('def')
insert into @names values ('g')
insert into @names values ('g')
insert into @names values ('g')