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 of Expressions, lists all the possible SQL expressions and indicates where they are allowed.
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 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. 

Constant 
Most builtin 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 SQL Boolean Operators. 

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.  
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.  
Builtin string functions 
The builtin 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. 