Expressions are generalized notions. Each expression contains several input parameters and returns an output result. An input parameter may be a constant or a single row of data, or multiple rows of data. Expressions can be combined. The input of one expression can be the output of another expression.
Expressions can be divided into the following categories based on the source and form:
SELECT ABS(a + 1) FROM t1 WHERE a > 0;
- a is a column reference.
- 0 and 1 are constants.
+are operators that take 0, 1, and the a expression as input.
ABSis a function that takes the
+expression as input.