All identifiers (including column names) that are not double-quoted are converted to lower case in PostgreSQL. Column names that were created with double-quotes and thereby retained upper-case letters (and/or other syntax violations) have to be double-quoted for the rest of their life. So, PostgreSQL column names are case-sensitive:
SELECT * FROM people WHERE "first_Name" = 'xyz';
'xyz' does not need to be double-quoted. Values (string literals) are enclosed in single quotes.
If you try to do this query ( first_Name is a column in the table people
SELECT * FROM people WHERE first_Name='xyz';
And you will meet this error
ERROR: column “first_Name” does not exist
Advice is to use legal, lower-case names exclusively so double-quoting is not needed.
(Thanks, Scott, for some hints.)