Question

I’m not sure how to ask this question in a way that Google will understand. I want to left join two tables and have the blank fields filled in. Here’s what I’m working with:

tbl1
| user | date  | usage |
| us1  | 01/12 |  503  |
| us1  | 02/12 |  245  |
| us1  | 03/12 |  465  |
| us2  | 02/12 |  327  |
| us2  | 03/12 |  204  |
| us3  | 02/12 |  156  |

tbl2
| dates | avg_use |
| 01/12 |   345   |
| 02/12 |   426   |
| 03/12 |   502   |

Desired output
| user | date  | usage  |  pct_of_avg  |
| us1  | 01/12 |  503   |     1.45     |
| us1  | 02/12 |  245   |     .58      |
| us1  | 03/12 |  465   |     .93      |
| us2  | 01/12 | (null) |      0       |
| us2  | 02/12 |  327   |     .95      |
| us2  | 03/12 |  204   |     .41      |
| us3  | 01/12 | (null) |      0       |
| us3  | 02/12 |  156   |     .37      |
| us3  | 03/12 | (null) |      0       |

I understand JOINs pretty well, so I’m aware that a typical JOIN will not be able to do this since the data in tbl1.user won’t exist. Is there a way to do this in SQL at all? Bonus points for giving what I’m trying to do a name, because it would help a lot just to know how to Google this 🙂

Answer 1

This should do it:

SELECT U.user, T2.dates date, T1.usage, T1.usage/T2.avg_use pct_of_avg  
FROM (SELECT DISTINCT user FROM tbl1) U
CROSS JOIN tbl2 T2
LEFT JOIN tbl1 T1
    ON T2.date = T1.date  AND U.user = T1.user