How to re-use result from a SELECT statement? - mysql

Question

My goal is to re-use the result from a SELECT statement to be used in SQL EXISTS statement.
The general idea looks like this:
SELECT *
FROM table
WHERE col1=1
OR EXISTS (
SELECT 1 // this is an exact copy of the SELECT statement above.
FROM table
WHERE col=1
)
The actual SQL statement I am trying to reduce:
"SELECT user_detail.user, user_detail.name, channel_member.role " +
"FROM user_detail " +
"JOIN channel_member ON user_detail.user=channel_member.user " +
"AND channel_member.uuid=#{channelUuid} " +
"WHERE user_detail.user=#{username} " +
"OR EXISTS ( " +
" SELECT 1" +
" FROM user_detail " +
" JOIN channel_member ON user_detail.user=channel_member.user " +
" AND channel_member.uuid=#{channelUuid} " +
" WHERE user_detail.user=#{username} " +
")"

Answer 1

You can only do this if your version of MySQL supports window functions, ie. version >= 8.0
You can use conditional window aggregation, like this:
SELECT *
FROM (
SELECT *, COUNT(CASE WHEN col1 = 1 THEN 1 END) OVER () AS CountMatches
FROM table
) t
WHERE CountMatches > 0;
Depending on the number of rows matching to non-matching, this may be more or less performant. You need to test.

Answer 2

This query:
SELECT *
FROM table
WHERE col1 = 1 OR
EXISTS (SELECT 1 // this is an exact copy of the SELECT statement above.
FROM table
WHERE col=1
)
Doesn't really make sense. It is saying to return all rows if col = 1 is in the table -- but then it filters to check if any row has col = 1. So it is equivalent to:
SELECT *
FROM table
WHERE (SELECT 1 FROM table t2 WHERE t2.col = 1);
I strongly suspect that you intend NOT EXISTS -- so get everything with 1. If there is no 1 then return everything:
SELECT *
FROM table
WHERE col1 = 1 OR
NOT EXISTS (SELECT 1 // this is an exact copy of the SELECT statement above.
FROM table
WHERE col = 1
);
This should work fine with tables -- and is in fact probably optimal with the right indexes.
If "table" is really a complex query, then you might consider window functions:
select t.*
from (select t.*,
sum( col = 1 ) as num_1s
from t
) t
where col = 1 or num_1s = 0;

Answer 3

If you want to use any conditional statement on the query you are running you will need to wrap the query and put it in a FROM statement and then run the conditional outside of the query, like so....
SELECT aliasName.*
FROM
(SELECT *
FROM table
WHERE col1=1) aliasName
WHERE EXISTS aliasName //this is the conditionl statement OUTSIDE of the query you built.
Let me know how you do...

Answer 4

For Mr./Ms. Barmar:
The idea
// Idea:
// If one of the row have col1 with value 1.
// Then return all of the row, or return empty []
// SELECT *
// FROM table
// WHERE col1=1
// OR EXISTS (
// SELECT 1
// FROM table
// WHERE col=1
// )
The solution
"WITH temp AS (" +
" SELECT user_detail.user, user_detail.name, channel_member.role " +
" FROM user_detail " +
" JOIN channel_member ON user_detail.user=channel_member.user " +
" AND channel_member.uuid=#{channelUuid} " +
") " +
"SELECT * " +
"FROM temp " +
"WHERE user=#{username} " +
" OR EXISTS ( " +
" SELECT 1" +
" FROM temp " +
" WHERE user=#{username} " +
" )"
The solution above use WITH clause as recommended by Mr./Ms. Barmar, I am posting this, so you can inspect whether this is logical or not.