About SQL Conditions

Conditions can have several forms, as shown in the following syntax.

condition::=

Text description of condition

The sections that follow describe the various forms of conditions. You must use appropriate condition syntax whenever condition appears in SQL statements.

You can use a condition in the WHERE clause of these statements:

• DELETE
• SELECT
• UPDATE

You can use a condition in any of these clauses of the SELECT statement:

• WHERE
• START WITH
• CONNECT BY
• HAVING

A condition could be said to be of the "logical" datatype, although Oracle does not formally support such a datatype.

The following simple condition always evaluates to TRUE:

1 = 1 

The following more complex condition adds the sal value to the comm value (substituting the value 0 for null) and determines whether the sum is greater than the number constant 2500:

NVL(salary, 0) + NVL(salary + (salary*commission_pct, 0) > 25000)

Logical conditions can combine multiple conditions into a single condition. For example, you can use the AND condition to combine two conditions:

(1 = 1) AND (5 < 7) 

Here are some valid conditions:

name = 'SMITH' 
employees.department_id = departments.department_id 
hire_date > '01-JAN-88' 
job_id IN ('SA_MAN', 'SA_REP') 
salary BETWEEN 5000 AND 10000
commission_pct IS NULL AND salary = 2100

Condition Precedence

Precedence is the order in which Oracle evaluates different conditions in the same expression. When evaluating an expression containing multiple conditions, Oracle evaluates conditions with higher precedence before evaluating those with lower precedence. Oracle evaluates conditions with equal precedence from left to right within an expression.

Table 5-1 lists the levels of precedence among SQL condition from high to low. Conditions listed on the same line have the same precedence. As the table indicates, Oracle evaluates operators before conditions.

Table 5-1 SQL Condition Precedence  

Type of Condition	Purpose
SQL operators are evaluated before SQL conditions	See "Operator Precedence"
=, !=, <, >, <=, >=,	comparison
IS [NOT] NULL, LIKE, [NOT] BETWEEN, [NOT] IN, EXISTS, IS OF type	comparison
NOT	exponentiation, logical negation
AND	conjunction
OR	disjunction