How to select rows with matching fields in MySQL? - mysql

Question

I have 2 tables, one with price overrides and one with minute overrides, each per user and product. How can I select them so that I have a record for each user and product?
Table 1: Override Prices
+--------+-----------+---------------+
| userID | productID | overridePrice |
+--------+-----------+---------------+
| 4 | 53 | 99.99 |
| 4 | 13 | 99.99 |
| 4 | 55 | 99.99 |
+--------+-----------+---------------+
Table 2: Override Minutes
+--------+-----------+---------------+
| userID | productID | overrideMin |
+--------+-----------+---------------+
| 4 | 18 | 23 |
| 4 | 55 | 4 |
| 50 | 55 | 2 |
+--------+-----------+---------------+
The table I want it to produce:
Table 2: All overrides
+--------+-----------+-------------+---------------+
| userID | productID | overrideMin | overridePrice |
+--------+-----------+-------------+---------------+
| 4 | 13 | null | 99.99 |
| 4 | 18 | 23 | null |
| 4 | 53 | null | 99.99 |
| 4 | 55 | 4 | 99.99 |
| 50 | 55 | 2 | null |
+--------+-----------+-------------+---------------+
I've attempted to GROUP BY userID, productID, but because product IDs may exist in table 1 that don't exist in table 2, I get different results depending on which productID I group by.

Answer 1

Use UNION:
SELECT userID, productID,
MAX(overrideMin) AS overrideMin,
MAX(overridePrice) AS overridePrice
FROM
(
SELECT userID, productID, null AS overrideMin, overridePrice
FROM OverridePrices
UNION
SELECT userID, productID, overrideMin, null AS overridePrice
FROM OverrideMinutes
) AS t
GROUP BY userID, productID;
This will give you the exact results you are looking for:
| userID | productID | overrideMin | overridePrice |
|--------|-----------|-------------|---------------|
| 4 | 13 | (null) | 99.99 |
| 4 | 18 | 23 | (null) |
| 4 | 53 | (null) | 99.99 |
| 4 | 55 | 4 | 99.99 |
| 50 | 55 | 2 | (null) |

Answer 2

You have to use IF statement:
SELECT
t1.userID,
t1.productID,
if (t2.overrideMin IS NULL, NULL, t2.overrideMin) AS overrideMin,
if (t1.overridePrice IS NULL, NULL, t1.overridePrice) AS overridePrice
FROM OverridePrices AS t1
LEFT JOIN OverrideMinutes AS t2 ON t1.userID = t2.userID AND t1.productID = t2.productID;
But in case you can have different products in table1 and table2 you have to join to table which contains all products, like:
SELECT
t1.userID,
t1.productID,
if (t2.overrideMin IS NULL, NULL, t2.overrideMin) AS overrideMin,
if (t1.overridePrice IS NULL, NULL, t1.overridePrice) AS overridePrice
FROM (
SELECT userID, productID FROM OverridePrices
UNION
SELECT userID, productID FROM OverrideMinutes
) AS t0
LEFT JOIN OverridePrices AS t1 ON t0.userID = t1.userID AND t0.productID = t1.productID
LEFT JOIN OverrideMinutes AS t2 ON t0.userID = t2.userID AND t0.productID = t2.productID;
Also, now you can have GROUP, HAVING etc.

Answer 3

SELECT table1.userID,table1.productID,table2.overrideMin,table1.overridePrice FROM table1
LEFT JOIN table2 ON table1.userID=table2.userID AND table1.productID = table2.productID
UNION SELECT table2.userID,table2.productID,table2.overrideMin,table1.overridePrice FROM table1
RIGHT JOIN table2 ON table1.userID=table2.userID AND table1.productID = table2.productID
ORDER BY userID, productID
This is the OUTPUT