SQL AST: ifnull/nullif improvements

[hemberger]

Previously the IfNullReturnTypeExtension was used for both IFNULL and NULLIF. However, these SQL functions should have different behavior.

NULLIF(expr1,expr2)

Returns NULL if expr1 = expr2 is true, otherwise returns expr1.

IFNULL(expr1,expr2)

If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2.

Note that IFNULL has additional type-mutating logic because it has to compare the two values in a temporary table:

The default return type of IFNULL(expr1,expr2) is the more "general" of the two expressions, in the order STRING, REAL, or INTEGER.

We now also require that the functions take exactly two arguments.

The typemix table is used for these tests since its column names clearly identify the column type (e.g. c_tinyint vs. gesperrt).

[staabm]

thanks for the in detail discussion and your analysis. I really appreciate it.