I believe there should be a simple way to do this but I'm stuck here! I've read 20+ pages on here and elsewhere but couldn't find what I want.
I have one single table with few thousands of rows and tens of columns, but to simplify let's imaging the following table qu is my data:
+----+-----+-----+------+
| id | PID | AID | Data |
+----+-----+-----+------+
| 1 | A | 1 | 56 |
| 2 | A | 2 | 234 |
| 3 | B | 1 | 23 |
| 4 | B | 2 | 78 |
| 5 | B | 3 | 65 |
| 6 | C | 2 | 89 |
| 7 | C | 3 | 74 |
+----+-----+-----+------+
I want to have a query that generates the following results:
+-----+-----+------+
| PID | AID | Data |
+-----+-----+------+
| A | 1 | 56 |
| A | 2 | 234 |
| A | 3 | NULL |
| B | 1 | 23 |
| B | 2 | 78 |
| B | 3 | 65 |
| C | 1 | NULL |
| C | 2 | 89 |
| C | 3 | 74 |
+-----+-----+------+
Basically I want the query to fill all the missing AID for PIDs and add NULL or NA for their Data values. I can achieve this by doing loops outside MySQL but it's very slow as I need to run individual queries for every single PID and AID combination to get the Data value.
Here is one of my latest tries with no success!
SELECT
*
FROM
(
SELECT
`id`,
`PID`
FROM
`qu`
GROUP BY
`PID`
) `a`
LEFT OUTER JOIN(
SELECT
`id`,
`AID`
FROM
`qu`
GROUP BY
`AID`
) `b`
ON
`a`.`id` = `b`.`id`
LEFT OUTER JOIN `qu` `c` ON
`a`.`id` = `c`.`id` AND `b`.`id` = `c`.`id`
You may use a calendar table approach here:
SELECT
q1.PID,
q2.AID,
q3.Data -- or use COALESCE(q3.Data, 'NA') AS Data
FROM (SELECT DISTINCT PID FROM qu) q1
CROSS JOIN (SELECT DISTINCT AID FROM qu) q2
LEFT JOIN qu q3
ON q3.PID = q1.PID AND
q3.AID = q2.AID
ORDER BY
q1.PID,
q2.AID;
The idea here is that we generate all possible combinations of PID and AID using the cross join between the distinct subqueries aliased as t1 and t2. Then, we left join with your actual qu table, which either brings in the data if available, or else NULL if not available.