Alias every table in a FROM clause and qualify the column references, also in single-table SELECTs. SQLite needs this because it reads an unknown, unqualified double-quoted column name as a string literal instead of failing.

Whether a NULL literal of type ti that is a column of a select list is written as cast(null as <type>). Without the cast, a database has to guess the type of the column, e.g. in a subquery or a union of NULLs, and PostgreSQL and HSQLDB guess one that does not match the other side of a union or the column's use in an outer query. A bind variable is never cast.

The cast's type is the default SQL type of ti, not the type of the other sides of a union. A union of strings, times or most numbers takes the largest length or precision of its sides, so the cast can only widen the result. A union of DECIMALs adds up the largest scale and the largest number of integer digits of its sides, so where that sum is capped (38 digits on SQL Server, 31 on DB2), the cast's DECIMAL(21,2) can cost a high-scale column its scale or integer digits. Those profiles leave a DECIMAL NULL untyped.

A String mapped onto a column of a custom database type, such as a PostgreSQL enum, has the SQL type VARCHAR, so its NULL is cast to VARCHAR and does not match the column in a union. Such a type needs a sqlTypeName that names the real type.

The name of the target table of the UPDATE or DELETE statement being built. A subquery that reads the same table must alias it, so that its columns don't resolve to the target table.

The maximum number of operands of an and or or that are rendered as one flat list. Longer chains are split into a balanced tree of parenthesized groups, so that their nesting depth in the database's SQL parser stays logarithmic (e.g. SQLite limits the expression depth to 1000 and Derby's SQL compiler recurses over it).