JOIN with GROUP BY in a Normalized DB about Resources, Topics & Chapters

Question

I\'ve normalized my DB but can\'t seem to return the data I\'m looking for in the right way.

I\'ve got 5 tables:

1. Resources (5 resources)
2. Topics (1

Answer 1

This is non-standard syntax:

SELECT *
...
GROUP BY RES.RES_ID

Here you have asked for every column (select *) but only specified one column under group by. ONLY MySQL allows this non-standard GROUP BY syntax which is controlled by server settings. Those default settings have recently changed and you may find many our queries using non-standard GROUP BY could fail in future.

A standards compliant GROUP BY clause specifies ALL "non-aggregating" columns. Meaning you must specify every column not using SUM/COUNT/AVG/MIN/MAX etc.

---

SELECT * FROM TOPICS, CHAPTERS, RESOURCES AS RES
INNER JOIN TOPICS_to_RESOURCE AS TR ON RES.RES_ID = TR.TR_RESID
INNER JOIN TOPICS_to_CHAPTER AS TCH ON TR.TR_TID = TCH.TCH_TID
GROUP BY RES.RES_ID

That query has 2 forms of join syntax. **FROM t1,t2,t3 ** is ancient and unwise and is NOT good practice. Also; never "combine" this old syntax form with the more recent join syntax in a single query as that can lead to query errors.

Just do NOT use commas between tables in the from clause this simple step will make you use the better syntax all he time.

By the way FROM TOPICS, CHAPTERS, RESOURCES AS RES with nothing to limit these in a where clause will produce:

Multiply every row in TOPICS by every row in CHAPTERS by every row in RESOURCES. In other words a "Cartesian product". In more recent syntax your query translates to:

SELECT * FROM TOPICS
CROSS JOIN CHAPTERS
CROSS JOIN RESOURCES AS RES
INNER JOIN TOPICS_to_RESOURCE AS TR ON RES.RES_ID = TR.TR_RESID
INNER JOIN TOPICS_to_CHAPTER AS TCH ON TR.TR_TID = TCH.TCH_TID
GROUP BY RES.RES_ID