Overview of expressions

An expression is a general concept. It usually contains one or more input parameters and returns an output result. The input parameters may come from constants or single-row data, or from multiple-row data. An expression can be a combination. The input of one expression can be the output of another expression.

Based on the source and form of an expression, it can be categorized into the following types:

• Column reference

• Constant

• Operator

• Function

An expression can be used in multiple locations in an SQL statement, such as in the ORDER BY or HAVING clause of a SELECT statement, in the WHERE clause of a SELECT, DELETE, or UPDATE statement, or in a SET statement. You can use values from multiple sources to write an expression. These sources include literals, column values, NULL, variables, built-in functions and operators, and user-defined functions and stored functions (a type of stored object).

Here is an example:

SELECT ABS(a + 1)
  FROM t1
 WHERE a > 0;

In the preceding example, the parameters are described as follows:

• a is a column reference.

• 0 and 1 are constants.

• > and + are operators. The operators take 0, 1, and a as input.

• ABS is a function. The function takes the + operator as input.