Mysql count id that appear in 2 columns - mysql

Question

I have a table with these columns:
id
user_id
player_in
player_out
date
I need to make a report that count the number of repetitions each "player" both in player_in field, as in player_out field.
For example, if I have this 2 rows in the table (in the respective order).
id user_id player_in player_out
1 1 88 56
2 7 77 88
The result for the player 88 will be 2, and for the players 56 and 77, just 1

Answer 1

Use a subquery that employs union all to get the two column into one column, then use a standard count(*):
Note: Thus query included individual totals for ins and outs as per further request in comments to this answer.
select
player_id,
count(*) as total,
sum(ins) as ins,
sum(outs) as outs
from (
select
player_in as player_id,
1 as ins,
0 as outs
from mytable
union all
select player_out, 0, 1
from mytable
) x
group by player_id
Note: you must use union all (not just union), because union removes duplicates whereas union all does not.

Answer 2

You could use a cross-join to a 2-row virtual table to unpivot the player_* columns, then group the results, like this:
SELECT
player,
COUNT(*) AS total_count
FROM (
SELECT
CASE WHEN x.is_in THEN t.player_in ELSE t.player_out END AS player
FROM mytable t
CROSS JOIN (SELECT TRUE AS is_in UNION ALL SELECT FALSE) x
) s
GROUP BY
player
;
That is, every row of the original table is essentially duplicated and each copy of the row supplies either player_in or player_out, depending on whether the derived table's is_in column is TRUE or FALSE, to form a single player column. This method of unpivoting might perform better than the UNION method suggested by #Bohemian because this way the (physical) table is passed just once (but you'd need to test and compare both methods to determine if there's any substantial benefit to this approach in your particular situation).
To calculate in and out counts, as you have requested in one of your comments to the above mentioned answer, you could extend my original suggestion like this:
SELECT
player,
COUNT( is_in OR NULL) AS in_count,
COUNT(NOT is_in OR NULL) AS out_count,
COUNT(*) AS total_count
FROM (
SELECT
x.is_in,
CASE WHEN x.is_in THEN t.player_in ELSE t.player_out END AS player
FROM mytable t
CROSS JOIN (SELECT TRUE AS is_in UNION ALL SELECT FALSE) x
) s
GROUP BY
player
;
As you can see, the derived table now additionally returns the is_in column in its own right, and the column is used in two conditional aggregations for counting how many times a player was in and out. (If you are interested, the OR NULL trick is explained here.)
You could also rewrite the COUNT(condition OR NULL) entries as SUM(condition). That would certainly shorten both expressions, some also find the SUM method of counting clearer/more elegant. In either event, there would likely be no difference in performance, so choose whichever method suits your taste better.
A SQL Fiddle demo of the second query can be found here.