A combination of symbols and operators that the database system evaluates to obtain a single data value. Simple expressions can be a single constant, variable, column, or scalar function. Operators can be used to join two or more simple expressions into a complex expression.
For a simple expression built of a single constant, variable, scalar function, or column name, the data type, precision, scale, and value of the expression is the data type, precision, scale, and value of the referenced element.
When two expressions are combined by using comparison or logical operators, the resulting data type is Boolean and the value is one of the following:
When two expressions are combined by using arithmetic, bitwise, or string operators, the resulting data type is determined by the operator.
Complex expressions made up of many symbols and operators evaluate to a single-valued result. The data type, precision, and value of the resulting expression are determined by combining the component expressions two at a time until a final result is reached. The sequence in which the expressions are combined is defined by the precedence of the operators in the expression.
An operator can combine two expressions if they both have data types supported by the operator and at least one of the following conditions is true:
The expressions have the same data type.
The data type with the lower precedence can be implicitly converted to the data type with the higher data type precedence.
If there is no supported implicit conversion, the two expressions cannot be combined.
In a programming language such as Microsoft Visual Basic, an expression always evaluates to a single result. Expressions in an SQL select list have a variation on this rule: The expression is evaluated individually for each row in the result set. A single expression might have a different value in each row of the result set, but each row has only one value for the expression. For example, in this SELECT statement, both the reference to ProductID and the term 1+2 in the select list are expressions.
SELECT [Product ID], 1 + 2 FROM Products
The expression 1+2 evaluates to 3 in each row in the result set. Although the expression ProductID generates a unique value in each result set row, each row has only one value for ProductID.