SQL expressions

Syntax for many statements and expressions includes the term expression, or a term for a specific kind of expression such as tableSubquery. Expressions are allowed in these specified places within statements.

Some locations allow only a specific type of expression or one with a specific property. If not otherwise specified, an expression is permitted anywhere the word expression appears in the syntax. This includes:

Of course, many other statements include these elements as building blocks, and so allow expressions as part of these elements.

The following tables list all the possible SQL expressions and indicate where the expressions are allowed.

General expressions

General expressions are expressions that might result in a value of any type. The following table lists the types of general expressions.

Table 1. General expressions
Expression Type	Explanation
Column reference	A `columnName` that references the value of the column made visible to the expression containing the Column reference. You must qualify the `columnName` by the table name or correlation name if it is ambiguous. The qualifier of a `columnName` must be the correlation name, if a correlation name is given to a table that is in a FROM clause. The table name is no longer visible as a columnName qualifier once it has been aliased by a correlation name. Allowed in selectExpressions, UPDATE statements, and the WHERE clauses of data manipulation statements.
Constant	Most built-in data types typically have constants associated with them (as shown in Data types).
NULL	NULL is an untyped constant representing the unknown value. Allowed in CAST expressions or in INSERT VALUES lists and UPDATE SET clauses. Using it in a CAST expression gives it a specific data type.
Dynamic parameter	A dynamic parameter is a parameter to an SQL statement for which the value is not specified when the statement is created. Instead, the statement has a question mark (?) as a placeholder for each dynamic parameter. See Dynamic parameters. Dynamic parameters are permitted only in prepared statements. You must specify values for them before the prepared statement is executed. The values specified must match the types expected. Allowed anywhere in an expression where the data type can be easily deduced. See Dynamic parameters.
CAST expression	Lets you specify the type of NULL or of a dynamic parameter or convert a value to another type. See CAST function.
Scalar subquery	Subquery that returns a single row with a single column. See scalarSubquery.
Table subquery	Subquery that returns more than one column and more than one row. See tableSubquery. Allowed as a tableExpression in a FROM clause and with EXISTS, IN, and quantified comparisons.
Conditional expression	A conditional expression chooses an expression to evaluate based on a boolean test. Conditional expressions include the CASE expression, the NULLIF function, and the COALESCE function.

Boolean expressions

Boolean expressions are expressions that result in boolean values. Most general expressions can result in boolean values. See Boolean expressions for more information and a table of operators.

Numeric expressions

Numeric expressions are expressions that result in numeric values. Most of the general expressions can result in numeric values. Numeric values have one of the following types:

BIGINT
DECIMAL
DOUBLE PRECISION
INTEGER
REAL
SMALLINT

The following table lists the types of numeric expressions.

Table 1. Numeric expressions
Expression Type	Explanation
+, -, *, /, unary + and - expressions	Evaluate the expected math operation on the operands. If both operands are the same type, the result type is not promoted, so the division operator on integers results in an integer that is the truncation of the actual numeric result. When types are mixed, they are promoted as described in Data types. Unary + is a noop (i.e., +4 is the same as 4). Unary - is the same as multiplying the value by -1, effectively changing its sign.
AVG	Returns the average of a set of numeric values. See AVG function.
SUM	Returns the sum of a set of numeric values. See SUM function.
LENGTH	Returns the number of characters in a character or bit string. See LENGTH function.
LOWER	See LCASE or LOWER function.
COUNT	Returns the count of a set of values. See COUNT function, COUNT(*) function.

Character expressions

Character expressions are expressions that result in a CHAR or VARCHAR value. Most general expressions can result in a CHAR or VARCHAR value. The following table lists the types of character expressions.

Table 1. Character expressions
Expression Type	Explanation
A CHAR or VARCHAR value that uses wildcards.	The wildcards % and _ make a character string a pattern against which the LIKE operator can look for a match.
Concatenation expression	In a concatenation expression, the concatenation operator, "\|\|", concatenates its right operand to the end of its left operand. Operates on character and bit strings. See Concatenation operator.
Built-in string functions	The built-in string functions act on a String and return a string. See LTRIM function, LCASE or LOWER function, RTRIM function, TRIM function, SUBSTR function, and UCASE or UPPER function.
USER functions	User functions return information about the current user as a String. See CURRENT_USER function, SESSION_USER function, and USER function.

Date and time expressions

A date or time expression results in a DATE, TIME, or TIMESTAMP value. Most of the general expressions can result in a date or time value. The following table lists the types of date and time expressions.

Table 1. Date and time expressions
Expression Type	Explanation
CURRENT_DATE	Returns the current date. See CURRENT_DATE function.
CURRENT_TIME	Returns the current time. See CURRENT_TIME function.
CURRENT_TIMESTAMP	Returns the current timestamp. See CURRENT_TIMESTAMP function.