How to combine near same item by SQL? - mysql

Question

I have some data in database:
id user
1 zhangsan
2 zhangsan
3 zhangsan
4 lisi
5 lisi
6 lisi
7 zhangsan
8 zhangsan
I want keep order, and combine near same user items, how to do it?
When I use shell script, I will(data in file test.):
cat test|cut -d " " -f2|uniq -c
this will get result as:
3 zhangsan
3 lisi
2 zhangsan
But how to do it use sql?

Answer 1

If you try:
SET #name:='',#num:=0;
SELECT id,
#num:= if(#name = user, #num, #num + 1) as number,
#name := user as user
FROM foo
ORDER BY id ASC;
This gives:
+------+--------+------+
| id | number | user |
+------+--------+------+
| 1 | 1 | a |
| 2 | 1 | a |
| 3 | 1 | a |
| 4 | 2 | b |
| 5 | 2 | b |
| 6 | 2 | b |
| 7 | 3 | a |
| 8 | 3 | a |
+------+--------+------+
So then you can try:
SET #name:='',#num:=0;
SELECT COUNT(*) as count, user
FROM (
SELECT #num:= if(#name = user, #num, #num + 1) as number,
#name := user as user
FROM foo
ORDER BY id ASC
) x
GROUP BY number;
Which gives
+-------+------+
| count | user |
+-------+------+
| 3 | a |
| 3 | b |
| 2 | a |
+-------+------+
(I called my table foo and also just used names a and b because I was too lazy to write zhangsan and lisi over and over).

Answer 2

if in oracle, you can do like below.
SELECT NAME,
num - lagnum
FROM (SELECT lagname,
NAME,
num,
nvl(lag(num) over(ORDER BY num), 0) lagnum
FROM (SELECT id,
lag(NAME) over(ORDER BY ID) lagname,
NAME,
lead(NAME) over(ORDER BY ID) leadname,
ROWNUM num
FROM (SELECT * FROM test ORDER BY ID))
WHERE (lagname = NAME AND (NAME <> leadname OR leadname IS NULL))
OR (lagname IS NULL AND NAME <> leadname)
OR (lagname <> NAME AND leadname IS NULL)
ORDER BY ID);

Answer 3

if in sql server, oracle, db2...
with x as(
select c.*, rn = row_number() over (order by c.id)
from test c
left join test n
on c.[user] = n.[user]
and c.[id] + 1 = n.[id]
where n.id is null
)
select a.[user], a.id - coalesce(b.id, 0)
from x a
left join x b
on a.rn = b.rn + 1

Answer 4

I think what you are looking for is to COUNT(ID):
SELECT COUNT(ID) FROM table GROUP BY user

Answer 5

You cannot do this in sql without doing some sort of sequential (iterative) analysis. Remember sql is set operation language.

Answer 6

A little improvement to the selected answer would be not to have to define those variables. So this query can be solved in just a single statement:
SELECT COUNT(*) cnt, user
FROM (
SELECT #num := #num + (#name != user) as number,
#name := user as user
FROM t, (select #num := 0, #name := '') as s
ORDER BY id
) x
GROUP BY number
Output:
| CNT | USER |
|-----|----------|
| 3 | zhangsan |
| 3 | lisi |
| 2 | zhangsan |
Fiddle here