How to auto increment a value in one table when inserted a row in another table

Question

I currently have two tables:

Table 1 has a unique ID and a count.

Table 2 has some data columns and one column where the value of the unique ID of Table 1 is inside.

When I insert a row of data in Table 2, the the count for the row with the referenced unique id in Table 1 should be incremented.
Hope I made myself clear. I am very new to PostgreSQL and SQL in general, so I would appreciate any help how to do that. =)

Answer 1

You could achieve that with triggers.
Be sure to cover all kinds of write access appropriately if you do. INSERT, UPDATE, DELETE.
Also be aware that TRUNCATE on Table 2 or manual edits in Table 1 could break data integrity.

I suggest you consider a VIEW instead to return aggregated results that are automatically up to date. Like:

CREATE VIEW tbl1_plus_ct AS
SELECT t1.*, t2.ct
FROM   tbl1 t1
LEFT JOIN  (
   SELECT tbl1_id, count(*) AS ct
   FROM   tbl2
   GROUP  BY 1
   ) t2 USING (tbl1_id)

If you use a LEFT JOIN, all rows of tbl1 are included, even if there is no reference in tbl2. With a regular JOIN, those rows would be omitted from the VIEW.

For all or much of the table, it is fastest to aggregate tbl2 first in a subquery, then join to tbl1 - like demonstrated above.

Instead of creating a view, you could also just use the query directly, and if you only fetch a single row, or only few, this alternative form would perform better:

SELECT t1.*, count(t2.tbl1_id) AS ct
FROM   tbl1 t1
LEFT   JOIN  tbl2 t2 USING (tbl1_id)
WHERE  t1.tbl1_id = 123  -- for example
GROUP  BY t1.tbl1_id     -- being the primary key of tbl1!