轉載自：http://blog.csdn.net/stevendbaguo/article/details/12611189
顯示那些佣金比例（commision）不大於20%或者爲NULL的員工的信息。
CREATE TABLE plch_employees
(
employee_id INTEGER PRIMARY KEY
, last_name VARCHAR2 (100)
, salary NUMBER
, commission_pct NUMBER
);
INSERT INTO plch_employees VALUES (100, 'Picasso', 1000000 , .3);
INSERT INTO plch_employees VALUES (200, 'Mondrian', 1000000, .15);
INSERT INTO plch_employeesVALUES (300, 'O''Keefe', 1000000, NULL);
COMMIT;
SQL> SELECT last_name,e.salary,e.commission_pct
FROM plch_employees e
WHERE e.commission_pct <= .2
OR e.commission_pct IS NULL;
LAST_NAME SALARY COMMISSION_PCT
--------------- ---------- --------------
Mondrian 1000000 .15
O'Keefe 1000000
SQL> SELECT last_name,e.salary,e.commission_pct
FROM plch_employees e
WHERE LNNVL (e.commission_pct > .2);
LAST_NAME SALARY COMMISSION_PCT
--------------- ---------- --------------
Mondrian 1000000 .15
O'Keefe 1000000
LNNVL 解釋：
LNNVL provides a concise way to evaluate a condition when one or both operands of the condition may be null. The function can be used only in the WHERE clause
of a query. It takes as an argument a condition and returns TRUE if the condition is FALSE or UNKNOWN and FALSE if
the condition is TRUE. LNNVL can be used anywhere a scalar expression can appear, even in contexts
where the IS [NOT] NULL, AND,
or OR conditions are not valid but would otherwise be required to account for potential nulls. Oracle Database sometimes uses the LNNVL function
internally in this way to rewrite NOT IN conditions as NOT EXISTS conditions.
In such cases, output from EXPLAIN PLAN shows this operation in the plan table output. The condition can
evaluate any scalar values but cannot be a compound condition containing AND, OR, or BETWEEN.
The table that follows shows what LNNVL returns given that a = 2 and b is null.
| Condition | Truth of Condition | LNNVL Return Value |
|---|---|---|
| a = 1 | FALSE |
TRUE |
| a = 2 | TRUE |
FALSE |
a IS NULL |
FALSE |
TRUE |
| b = 1 | UNKNOWN |
TRUE |
b IS NULL |
TRUE |
FALSE |
| a = b | UNKNOWN |
TRUE |
大致的意思是：lnnvl用於某個語句的where子句中的條件，如果條件爲true就返回false；如果條件爲UNKNOWN或者false就返回true。該函數不能用於複合條件如AND, OR, or BETWEEN中。