In MS SQL Server, with ANSI_NULLS set to OFF, it will select all rows having a NULL value for col. AFAIK not used that often.

That is a serious gotcha of SQL Server. It (and Oracle) has some serious problems with unique constraints because of that kind of stuff.

Seems like a good thing to check for! So we added a rule for this: https://github.com/sqlfluff/sqlfluff/pull/1527

Including autofixing if wanted.

Will be in the next release.

I guess it depends on what you want out of a linter e.g. if you want it to check just syntax or semantics as well. Is “= NULL” syntactically correct?

A linter is supposed to tell you "you are probably making a mistake."

"= NULL" is as syntactically correct in SQL as "if (x = null)" in C.

If it wasn't, code highlighting or compiling would break, both of which are much much more obvious.

The compiler checks syntax - if that’s all a linter did it wouldn’t be very useful.

`= NULL` is perfectly legal syntax - but it isn't doing what you think it is.

That's what parent is saying. He is contrasting a compiler (which this isn't) to a linter (which this arguably also isn't).