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This repository was archived by the owner on Jan 28, 2021. It is now read-only.
FR: Select single fields from substring search or split results #747
Description
I would like to be able to split a string and select one of the fields. The SPLIT function returns a JSON array, so I can't use that directly. There is a SUBSTRING function, but there does not appear to be any way to find the offset of a partition (what MySQL calls LOCATE or POSITION).
One way to address this would be to give SPLIT an optional third parameter, so that
mysql> SELECT SPLIT('a,b,c', ',');
'["a","b","c"]'
mysql> SELECT SPLIT('a,b,c', ',', 2);
'b'
Another would be to add a LOCATE (or FIND or POSITION or INDEX) function to find the offset of a subfield, e.g.,
mysql> SELECT LOCATE('a,b,c', ',');
2
mysql> SELECT SUBSTRING('a,b,c', 0, LOCATE('a,b,c', ',') - 1);
'a'
The first is probably better, since LOCATE would probably want a "skip" parameter anyway to specify which of the matches to stop on. But the second would also work.
Activity
smola commentedon Apr 10, 2019
We could implement
SUBSTRING_INDEX, which is supported both in MySQL and Spark SQL.It would be:
For second part, it's a quite verbose but still possible:
ajnavarro commentedon Apr 10, 2019
@creachadair Right now you can use
JSON_EXTRACTto get the value of a specific position:@smola having this implemented is it worth it implement
SUBSTRING_INDEX?smola commentedon Apr 10, 2019
@ajnavarro Since we have
JSON_EXTRACT, I see no urgency to implementSUBSTRING_INDEX, but we should still consider it when it comes to align a subset of functions between gitbase and Spark SQL. That is, those functions we want to be able to push down from Spark SQL.creachadair commentedon Apr 15, 2019
I have run into another case where
JSON_EXTRACTcan be a little limiting. Suppose I would like to identify branch points, that is to say, commits that have multiple children. This is not that interesting in a single repo, but is a useful basis for finding places where forks happened and diverged across multiple repos.The obvious way to do this is to do an autojoin on
commits, butcommit_parentsis multi-valued. You can filter by length withARRAY_LENGTH, butJSON_EXTRACTprojects the literal string value including its quotes, so you can't join on it without doing extra string surgery.I could write a program, but it seems unfortunate that we don't have a good way to get properly-typed values out of these denormalized fields. Arguably this is better solved with a schema change, but that's probably a lot more work (and maybe not worth it—this problem is mostly a demonstration case for me, not a blocking work item).
Anyway, just another datum on that while I'm working with it.
ajnavarro commentedon Apr 16, 2019
maybe implementing JSON inline selectors: https://dev.mysql.com/doc/refman/8.0/en/json-search-functions.html#operator_json-inline-path or just
JSON_UNQUOTEat the beginning we can fix that problem and many others.