When is comparing strings case-sensitive and when case-insensitive in SQL? - sql

Question

When is comparing strings case-sensitive and when case-insensitive in popular databases such as PostgreSQL, MySQL, MariaDB, Oracle, SQL Server, and SQLite?
I mean comparison using operators like: 'ab' = 'AB', or comparison of strings performed inside string functions like: POSITION('b' IN 'ABC'), INSTR('ABC', 'b'), REPLACE('ABC', 'b', 'x'), TRANSLATE('ABC', 'b', 'x'), TRIM('XabcX', 'x').
I think that I can know the answer but I don't know if it is correct.
Additionally, does the SQL standard define the case-sensitivity of string comparison?
Unfortunately, I only found a question about case-sensitivity of SQL syntax, and not in string comparison.
Edit: I am asking about default setting of the RDBMS, without additional setting the collation of a database, table, nor column.
I am asking only about ASCII letters A-Z and a-z.

Answer 1

Oracle DB functions/operators are case sensitive, with some exceptions including regexp_like method that can be switched to case-sensitive mode ;
--case sensitive demo
select q.txt
from (select 'myPhrase' txt from dual) q
where regexp_like(q.txt, 'myphrase'); --empty result set
select q.txt
from (select 'myPhrase' txt from dual) q
where q.txt like 'myphrase'; --empty result set
select q.txt
from (select 'myPhrase' txt from dual) q
where q.txt like 'myPhrase'; --exact match; returns value
select q.txt
from (select 'myPhrase' txt from dual) q
where regexp_like(q.txt, 'myphrase', 'i'); --case insensitive switch 'i'; returns value
select q.txt
from (select 'myPhrase' txt from dual) q
where lower(q.txt) like 'myphrase'; --enforced match; returns value;
Here's reference to docs for Collation rules for different SQL Operations and for Data Type Comparison Rules
Once a pseudo-collation is determined as the collation to use, NLS_SORT and NLS_COMP session parameters are checked to provide the actual named collation to apply
Usually these two are set to BINARY
SELECT
t.parameter,
t.value
FROM
nls_database_parameters t
WHERE
t.parameter IN (
'NLS_COMP', 'NLS_SORT'
);