MYSQL - 从多行，相同用户，不同值中选择

Question

Below is my table called fittest. I need to find which students based on student id (studid) have taken the pre and post test as designated in the prepost column. So based on the simple table below I would need to return studid 123456. How do I write the SELECT query for this?

SQL query: SELECT studid, prepost FROM `fittest` LIMIT 0, 30 ; 

    studid  prepost
    123456  pre
    123456  post
    1031460 pre

Answer 1

What about a JOIN:

select s1.studid, s1.prepost, s2.prepost
from fittest s1
inner join fittest s2
    on s1.studid = s2.studid
where s1.prepost = 'pre' 
  and s2.prepost = 'post'

Answer 2

Try

SELECT studid
  FROM fittest
 GROUP BY studid
HAVING COUNT(DISTINCT prepost) = 2

or if you want to ensure that only ids that have exactly one row with pre and one row with post then you can enforce it like this

SELECT studid
  FROM fittest
 GROUP BY studid
HAVING (MAX(prepost = 'pre' ) + 
        MAX(prepost = 'post')) = 2
   AND COUNT(DISTINCT prepost) = 2

Output:

| STUDID |
----------
| 123456 |

Here is SQLFiddle demo for both queries

Answer 3

try:

    select f1.studid
       from fittest f1
       inner join fittest f2
         on f1.studid = f2.studid
    where f1.prepost = 'pre' and f2.prepost = 'post'

here's the SQL Fiddle

Answer 4

Depending on your real Primary Key, you use the following:

SELECT studid
FROM fittest
WHERE prepost = 'pre' OR prepost = 'post'
GROUP BY studid
HAVING COUNT(DISTINCT prepost) = 2

Answer 5

SELECT t1.studid
FROM fittest t1
INNER JOIN t2 ON t1.studid=t2.studid
WHERE t1.prepost = 'pre'
AND t2.prepost='post'

Answer 6

If Only PRE and POST are possible in this column then try

SELECT studid FROM fittest group by studid
HAVING Min(prepost)<>max(prepost)

Answer 7

SELECT 
studid 
FROM `fittest` 
where prepost = 'pre' 
or prepost = 'post'
group by studid
having count(distinct prepost)=2

SQL Fiddle with Extra records.

Answer 8

You could use JOIN

SELECT a.studid 
FROM fittest a 
JOIN fittest b 
    ON a.studid=b.studid 
        and a.prepost like 'pre' 
        and b.prepost like 'post'

Answer 9

选择.... where studid in（123456,123457，...）