Simple psql count query - sql

Question

I am very new to postgresql and would like to generate some summary data from our table
We have a simple message board - table name messages which has an element ctg_uid. Each ctg_uid corresponds to a category name in the table categories.
Here are the categories select * from categories ORDER by ctg_uid ASC;
ctg_uid | ctg_category | ctg_creator_uid
---------+--------------------+-----------------
1 | general | 1
2 | faults | 1
3 | computing | 1
4 | teaching | 2
5 | QIS-FEEDBACK | 3
6 | QIS-PHYS-FEEDBACK | 3
7 | SOP-?-CHANGE | 3
8 | agenda items | 7
10 | Acq & Process | 2
12 | physics-jobs | 3
13 | Tech meeting items | 12
16 | incident-forms | 3
17 | ERRORS | 3
19 | Files | 10
21 | QIS-CAR | 3
22 | doses | 4
24 | admin | 3
25 | audit | 3
26 | For Sale | 4
31 | URGENT-REPORTS | 4
34 | dt-jobs | 3
35 | JOBS | 3
36 | IN-PATIENTS | 4
37 | Ordering | 4
38 | dep-meetings | 4
39 | reporting | 4
What I would like to do is for all messages on our messages is count the frequency of each category
I can do it on a category by category basis
SELECT count(msg_ctg_uid) FROM messages where msg_ctg_uid='13';
However is it possible to do this in a one liner?
The following gives the the category and ctg_uid for each message
SELECT ctg_category, msg_ctg_uid FROM messages INNER JOIN categories ON (ctg_uid = msg_ctg_uid);
but SELECT ctg_category, count(msg_ctg_uid) FROM messages INNER JOIN categories ON (ctg_uid = msg_ctg_uid);
gives me the error ERROR: column "categories.ctg_category" must appear in the GROUP BY clause or be used in an aggregate function
How do I aggregate the frequency of each category ?

Answer 1

You're missing the group by clause:
SELECT ctg_category, count(msg_ctg_uid)
FROM messages INNER JOIN categories ON (ctg_uid = msg_ctg_uid);
GROUP BY ctg_category
this means you want the count per ctg_category