如何合并具有相同日期和参与者ID（MYSQL）的所有行

Question

So I'm trying to write a simple mysql migration like script.
My goal is to select participant_id and date and if they = the same - return a count.
In this example the count would return 4.
I would then take the count and store it in a variable to be placed into another table.
Basically I'm consolidating the data from 4 rows into 1 row in a new table that will have the a count field.

My data currently.

ID   PARTICIPANT_ID  DATE  
---  --------------  ---------------------
"1"  "4"             "2015-05-06 21:35:37"
"2"  "4"             "2015-05-06 21:35:37"
"3"  "4"             "2015-05-06 21:35:37"
"4"  "4"             "2015-05-06 21:35:37"

My goal.

ID    PARTICIPANT_ID  DATE                   COUNT  
----  --------------  ---------------------  -----
"1"   "4"             "2015-05-06 21:35:37"  "4"

I've attempted writing a query like this:

SELECT ID, PARTICIPANT_ID, DATE FROM mytable

What changes do I need to make to my query to return the specified resultset?

Answer 1

Here's one (of several alternatives) that will return the specified result.

SELECT MIN(t.id) AS id
     , t.participant_id
     , t.date
     , COUNT(*) AS `count`
  FROM mytable t
 GROUP BY t.participant_id, t.date

The "trick" is the GROUP BY clause. That "collapses" all rows with common values for the listed expressions into a single row. The COUNT(*) aggregate function returns a count of rows that were "collapsed" into the GROUP . The MIN(t.id) aggregate returns the lowest value of the id column from rows in the group.

Answer 2

INSERT INTO TABLE2
select null, PARTICIPANT_ID, DATE, COUNT(*) from TABLE1
GROUP BY null, PARTICIPANT_ID, DATE

when ID-Field ist AUTOINCREMENT in TABLE2 he should set it correkt.

TIP: Don't name your columns "date" or "count" because these names CAN be keywords and you will get some trouble later.

Answer 3

the problem is only the first column id but you can do MIN(id) or MAX(id) if need.

SELECT participant_id, `date`, CONT(*)
FROM table1
GROUP BY participant_id, `date`

Answer 4

SELECT participant_id,date,count(*)
FROM yourtable
GROUP BY participant_id, date