sql - Selecting one of multiple matching rows with non-unique key

Question

I'm having an issue where I need to connect a unique key from one table that has multiple matching keys in another table, but I only need the most recent entry for each key.

The issue arises as I'm using tags to identify animals, however over time I need to re-use the tags. To solve this I use a unique key for each tag where I include the year of birth in the ID of the animal.

I now have some new recordings where I identify the sex of each animal, but only the tag is recorded. I need to match the tag to the correct unique animal tag, however as most of the tags are re-used I need to ensure I'm only matching the tag to the newest version, ie the most recent year for the relevant tag in the unique_tag column.

Is there a way to tell SQL which entry to use when there are multiple matching keys?

Simplified example tables and current code:

table_a:

unique_tag
2020TAG1
2016TAG2
2004TAG1
2001TAG2

table_b:

tag	sample_date	sex
TAG1	2022-02-10	M
TAG2	2022-02-10	F

SELECT unique_tag, table_b.tag, table_b.sample_date, table_b.sex
FROM table_a
INNER JOIN table_b
ON RIGHT(table_a.unique_tag, 4) = table_b.tag;

This code results in multiple entries for each tag, however I only need the one with the most recent year in the unique_tag.

Any suggestions are greatly appreciated!

Answer 1

You can use ORDER BY sample_date DESC LIMIT 1 this should give you one result where te date is the latest.

Answer 2

You can use row_number() and get the rows with the biggest year of tag for each tag:

SELECT unique_tag, tag, sample_date, sex FROM
(SELECT unique_tag, table_b.tag, table_b.sample_date, table_b.sex,
row_number() over(partition by RIGHT(table_a.unique_tag, 4) order by LEFT(table_a.unique_tag, 4) desc) rn
FROM table_a
INNER JOIN table_b
ON RIGHT(table_a.unique_tag, 4) = table_b.tag) t
WHERE rn = 1;

Fiddle

Answer 3

You can use a subquery with IN() condition:

SELECT unique_tag, table_b.tag, table_b.sample_date, table_b.sex
FROM table_a
INNER JOIN table_b
ON RIGHT(table_a.unique_tag, 4) = table_b.tag
WHERE table_b.tag IN( SELECT tag FROM table_b GROUP BY tag ORDER BY sample_date DESC);