Oracle函数

-- EXTRACT()函数：返回具体的年、月、日、时、分、秒等具体时间。
SELECT EXTRACT (YEAR FROM --SYSDATE
       SYSTIMESTAMP
       ) YEAR FROM DUAL;
       
SELECT EXTRACT (MONTH FROM --SYSDATE
       SYSTIMESTAMP
       ) YEAR FROM DUAL;
       
SELECT EXTRACT (DAY FROM --SYSDATE
       SYSTIMESTAMP
       ) YEAR FROM DUAL;
       
SELECT EXTRACT (HOUR FROM SYSTIMESTAMP) YEAR FROM DUAL;

SELECT EXTRACT (MINUTE FROM SYSTIMESTAMP) YEAR FROM DUAL;

SELECT EXTRACT (SECOND FROM SYSTIMESTAMP) YEAR FROM DUAL;

SELECT EXTRACT (DAY FROM SYSTIMESTAMP) YEAR FROM DUAL;

SELECT EXTRACT(YEAR FROM ORDERDATE) AS O

--EG:
SELECT EXTRACT(YEAR FROM SYSDATE) AS YEAR_,
       EXTRACT(MONTH FROM SYSDATE) AS MONTH_,
       EXTRACT(DAY FROM SYSDATE) AS DAY_
       FROM DUAL

--NVL(EXPR1,EXPR2)
--如果EXPR1不为空则返回EXPR1,否则返回EXPR2。

--NVL2(EXPR1,EXPR2)
--如果EXPR1不为空，返回EXPR2,否则返回EXPR3。

--说明：NVL()函数中的两个表达式的类型可以是：数字型，字符型，日期型。
--但必须是相同的类型，否则要对其进行类型转换。NVL(TO_CHAR(COLUMN_NAME),TO_CHAR(COLUMN_NAME)
--EG: SELECT NVL([COLUMN_NAME]),TO_CHAR([COLUMN_NAME])) FROM [TABLE_NAME];

--DECODE(EXPR1,IF1,VALUE1,IF2,VALUE2,IF3,VALUE3,VALUE4)
--DECODE()函数中如果EXPR1=IF1 RETURN VALUE1,ELSE EXPR1=IF2 RETURN VALUE2,ELSE EXPR1=IF3 RETURN VALUE3
--如果出现基数且EXPR1与所有的IF都不想等则直接返回基数列的值。
--EG:SELECT DECODE([COLUMN_NAME],'1','是','2','否','3','进行中','已结束') FROM [TABLE_NAME];