calculate percentages with postgresql join queries - percentage

Question

I am trying to calculate percentages by joining 3 tables data to get the percentages of positive_count, negative_count, neutral_count of each user's tweets. I have succeeded in getting positive, negative and neutral counts, but failing to get the same as percentages instead of counts. Here is the query to get counts:
SELECT
t1.u_id,count() as total_tweets_count ,
(
SELECT count() from t1,t2,t3 c
WHERE
t1.u_id='18839785' AND
t1.u_id=t2.u_id AND
t2.ts_id=t3.ts_id AND
t3.sentiment='Positive'
) as pos_count ,
(
SELECT count() from t1,t2,t3
WHERE
t1.u_id='18839785' AND
t1.u_id=t2.u_id AND
t2.ts_id=t3.ts_id AND
t3.sentiment='Negative'
) as neg_count ,
(
SELECT count() from t1,t2,t3
WHERE
t1.u_id='18839785' AND
t1.u_id=t2.u_id AND
t2.ts_id=t3.ts_id AND
t3.sentiment='Neutral'
) as neu_count
FROM t1,t2,t3
WHERE
t1.u_id='18839785' AND
t1.u_id=t2.u_id AND
t2.ts_id=t3.ts_id
GROUP BY t1.u_id;
**OUTPUT:**
u_id | total_tweets_count | pos_count | neg_count | neu_count
-----------------+--------------------+-----------+-----------+-------
18839785| 88 | 38 | 25 | 25
(1 row)
Now I want the same in percentages instead of counts. I have written the query in the following way but failed.
SELECT
total_tweets_count,pos_count,
round((pos_count * 100.0) / total_tweets_count, 2) AS pos_per,neg_count,
round((neg_count * 100.0) / total_tweets_count, 2) AS neg_per,
neu_count, round((neu_count * 100.0) / total_tweets_count, 2) AS neu_per
FROM (
SELECT
count(*) as total_tweets_count,
count(
a.u_id='18839785' AND
a.u_id=b.u_id AND
b.ts_id=c.ts_id AND
c.sentiment='Positive'
) AS pos_count,
count(
a.u_id='18839785' AND
a.u_id=b.u_id AND
b.ts_id=c.ts_id AND
c.sentiment='Negative'
) AS neg_count,
count(
a.u_id='18839785' AND
a.u_id=b.u_id AND
b.ts_id=c.ts_id AND
c.sentiment='Neutral') AS neu_count
FROM t1,t2, t3
WHERE
a.u_id='18839785' AND
a.u_id=b.u_id AND
b.ts_id=c.ts_id
GROUP BY a.u_id
) sub;
Can anyone help me out in achieving as percentages for each user data as below?
u_id | total_tweets_count | pos_count | neg_count | neu_count
------------------+--------------------+-----------+-----------+-----
18839785| 88 | 43.18 | 28.4 | 28.4
(1 row)

Answer 1

I am not entirely sure what you are looking for.
For starters, you can simplify your query by using conditional aggregation instead of three scalar subqueries (which btw. do not need to repeat the where condition on a.u_id)
You state you want to "count for all users", so you need to remove the WHERE clause in the main query. The simplification also gets rid of the repeated WHERE condition.
select u_id,
total_tweets_count,
pos_count,
round((pos_count * 100.0) / total_tweets_count, 2) AS pos_per,
neg_count,
round((neg_count * 100.0) / total_tweets_count, 2) AS neg_per,
neu_cont,
round((neu_count * 100.0) / total_tweets_count, 2) AS neu_per
from (
SELECT
t1.u_id,
count(*) as total_tweets_count,
count(case when t3.sentiment='Positive' then 1 end) as pos_count,
count(case when t3.sentiment='Negative' then 1 end) as neg_count,
count(case when t3.sentiment='Neutral' then 1 end) as neu_count
FROM t1
JOIN t2 ON t1.u_id=t2.u_id
JOIN t3 t2.ts_id=t3.ts_id
-- no WHERE condition on the u_id here
GROUP BY t1.u_id
) t
Note that I replaced the outdated, ancient and fragile implicit joins in the WHERE clause with "modern" explicit JOIN operators
With a more up-do-date Postgres version, the expression count(case when t3.sentiment='Positive' then 1 end) as pos_count can also be re-written to:
count(*) filter (where t3.sentiment='Positive') as pos_count
which is a bit more readable (and understandable I think).
In your query you can achieve the repetition of the global WHERE condition on the u_id by using a co-related subquery, e.g.:
(
SELECT count(*)
FROM t1 inner_t1 --<< use different aliases than in the outer query
JOIN t2 inner_t2 ON inner_t2.u_id = inner_t1.u_id
JOIN t3 inner_t3 ON inner_t3.ts_id = inner_t2.ts_id
-- referencing the outer t1 removes the need to repeat the hardcoded ID
WHERE innter_t1.u_id = t1.u_id
) as pos_count
The repetition of the table t1 isn't necessary either, so the above could be re-written to:
(
SELECT count(*)
FROM t2 inner_t2
JOIN t3 inner_t3 ON inner_t3.ts_id = inner_t2.ts_id
WHERE inner_t2.u_id = t1.u_id --<< this references the outer t1 table
) as pos_count
But the version with conditional aggregation will still be a lot faster than using three scalar sub-queries (even if you remove the unnecessary repetition of the t1 table).