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SQL Expressions

SQL Expressions

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. Table 7, lists all the possible SQL expressions and indicates where they are allowed.

If not otherwise specified, an expression is permitted anywhere the word Expression appears in the syntax. This includes:

SelectExpression
UPDATE statement (SET portion)
VALUES expression
WHERE clause

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

Table 7. Table of Expressions

	Expression Type	Explanation
General expressions		All expressions that might result in a value of any type.
	Column reference Allowed in SelectExpressions, UPDATE statements, and the WHERE clauses of data manipulation statements.	A column-Name that references the value of the column made visible to the expression containing the Column reference. You must qualify the column-Name by the table name or correlation name if it is ambiguous. The qualifier of a column-Name 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 column-Name qualifier once it has been aliased by a correlation name.
	Constant	Most built-in data types typically have constants associated with them (as shown in Data Types).
	NULL 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.	NULL is an untyped constant representing the unknown value.
	Dynamic parameter Allowed anywhere in an expression where the data type can be easily deduced. See Dynamic Parameters.	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.
	CAST expression	Lets you specify the type of NULL or of a dynamic parameter or convert a value to another type. See CAST.
	scalar subquery	Subquery that returns a single row with a single column. See ScalarSubquery.
	table subquery Allowed as a tableExpression in a FROM clause and with EXISTS, IN, and quantified comparisons.	Subquery that returns more than one column and more than one row. See TableSubquery.
	Conditional expression	A conditional expression chooses an expression to evaluate based on a boolean test.
Boolean expressions		Expressions that result in boolean values. Most general expressions can result in boolean values. Boolean expressions commonly used in a WHERE clause are made of operands operated on by SQL operators. See Table 8.
Numeric expressions		Expressions that result in numeric values. Most of the general expressions can result in numeric values. Numeric values have one of the following types: SMALLINT, INTEGER, BIGINT, REAL, DOUBLE PRECISION, DECIMAL.
	+, -, *, /, unary + and - expressions	+, -, *, /, unary + and - 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. AVG
	SUM	Returns the sum of a set of numeric values. SUM
	LENGTH	Returns the number of characters in a character or bit string. See LENGTH.
	LOWER	See LCASE or LOWER.
	COUNT	Returns the count of a set of values. See COUNT, COUNT(*).
Character expressions		Expressions that result in a CHAR or VARCHAR value. Most general expressions can result in a CHAR or VARCHAR value.
	A CHAR or VARCHAR value that uses wildcards. Used in a LIKE pattern.	The wildcards % and _ make a character string a pattern against which the LIKE operator can look for a match. See LIKE.
	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.
	Built-in string functions	The built-in string functions act on a String and return a string. See LTRIM, LCASE or LOWER, RTRIM, SUBSTR, and UCASE or UPPER
	USER functions	User functions return information about the current user as a String. See CURRENT_USER, SESSION_USER, and USER
Date/time expressions		A date/time expression results in a DATE, TIME, or TIMESTAMP value. Most of the general expressions can result in a date/time value.
	CURRENT_DATE	Returns the current date. See CURRENT_DATE.
	CURRENT_TIME	Returns the current time. See CURRENT_TIME.
	CURRENT_TIMESTAMP	Returns the current timestamp. See CURRENT_TIMESTAMP.

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