[PATCH] OGR SQL 'COUNT([DISTINCT] xx)' not conformant with other SQL implementations w.r.t NULL values

[Even Rouault]

Currently, OGR SQL considers COUNT(a_column) as a synonym of COUNT(*). However the standard mandates that COUNT(a_column) only count non-NULL values, whereas COUNT(*) count all rows (after the effect of WHERE clause of course). So COUNT(a_column) should not be considered as equivalent to COUNT(*)

The same applies for COUNT(DISTINCT a_column): it should only count unique non-NULL values.

References :

• ​http://www.w3schools.com/sql/sql_func_count.asp
• ​http://msdn.microsoft.com/en-us/library/ms175997.aspx
• ​http://www.sqlite.org/lang_aggfunc.html

What is a bit annoying here is that ogr_sql.html says that : "As a special case, the COUNT() operator can be given a "*" argument instead of a field name which is a short form for count all the records though it would get the same result as giving it any of the column names [...]"

If we fix the current behaviour to be conformant with the standard, it can break assumptions made by users. But currently, there's an inconsistency if you look at the result of -sql "select sum(foo), count(foo), avg(foo) from bar" when there are null values