T-SQL：计算列中每个唯一 substring 的出现次数

Question

I want to count occurrences per unique substring in a column.

SELECT DISTINCT LEFT(code, 3) 
FROM table-with-codes

Output:

code
------------------
VJCrandomthings
PASrandomthings
CAArandomthings
PASrandomthings2
PASrandomthings3

Expected output:

caa 1
pas 3
vjc 1

I tried

SELECT COUNT(DISTINCT LEFT(code, 3)) 
FROM table-with-codes 

but that returns 3 as result.

Answer 1

You need to count with group by

select Left(code,3), Count(*)
from [table-with-codes]
group by Left(code,3)

Answer 2

You are correct with using left but instead of distinct you should group by

declare @table_with_codes table (code varchar(50))
insert into @table_with_codes values ('VJCrandomthings'), ('PASrandomthings'), ('CAArandomthings'), ('PASrandomthings2'), ('PASrandomthings3')

select left(code, 3) as code_3, 
       count(*) as cnt
from   @table_with_codes 
group by left(code, 3)

how this works
By using the group by you can count all rows that are grouped (and thus only returned once in the result), in this case all rows with the same value in lef(col, 3)

The result will be

code_3  cnt
------  ---
CAA      1
PAS      3
VJC      1

Answer 3

You should keep the extracted 3 letters in the select statement and Group by .

Query

Select left(code, 3) as code3, count(left(code, 3)) as cnt
From tablename
Group by left(code, 3)
Order by 1;

Answer 4

I don't like having LEFT(code, 3) appear more than once in the query. To improve maintainability and readability I'd refactor it using CROSS APPLY, like this:

SELECT subcode, COUNT(*)
FROM table-with-codes
CROSS APPLY (SELECT LEFT(code, 3)) AS T(subcode)
GROUP BY subcode

Answer 5

You can use Common Transaction Expression to avoid mentioning left function two times:

create table [table-with-codes] (code nvarchar(100))
go
insert into [table-with-codes] values('VJCrandomthings'),('PASrandomthings'),('CAArandomthings'),('PASrandomthings2'),('PASrandomthings3');


with cte as (select LEFT(code,3) mycode from [table-with-codes])
select mycode,count(1) N from cte group by mycode