An expression, which specifies a value, can consist of literals (or constants), name references, or operations using names and literals.
Scalar Expressions
A scalar expression produces a single number, character string, byte string, date, time, timestamp, or interval.
A value expression has exactly one declared type common to every possible result of evaluation. Implicit type conversion rules apply to expressions.
Query Expressions
Query expressions operate on table values and produce rows and tables of data. Query expressions can include a FROM clause, which operates on a table reference and returns a single-table value.
Zero-table SELECT statements do not require a FROM clause.