FIREBIRD/INTERBASE内置函数使用说明

加*号为FB2.0加入的函数

整理:剑雷(jianlei) 2006-10-13

1. COUNT, AVG, MAX, MIN, SUM

说明:通用统计函数,不详细介绍了

2. EXTRACT(timestamp_part FROM value)

说明:EXTRACT(YEAR/MONTHE/DAY/WEEKDAY FROM 字段名)

从日期型字段中分离出年,月,日及一个星期的第几天

3. CAST(value AS datatype)

说明:转换数据类型

4. LOWER() *

说明:返回小写值

5. UPPER()

说明:返回大写值

6. TRIM() *

说明:去除字符串两边的空格

7. SUBSTRING(string FROM pos FOR length)

说明:取字符串子串,注意,第一个字符的位置是1

8. BIT_LENGTH *

说明:返回字符串位(bit)数

9. CHAR_LENGTH/CHARACTER_LENGTH *

说明:返回字符串字符数

10. OCTET_LENGTH *

说明:返回字符串字节数

11. CASE

说明:通过执行外来的一组条件取得相应的返回值

举例

i) 简单

SELECT o.ID, o.Description,

CASE o.Status

WHEN 1 THEN "confirmed"

WHEN 2 THEN "in production"

WHEN 3 THEN "ready"

WHEN 4 THEN "shipped"

ELSE "unknown status """ || o.Status || """"

END

FROM Orders o;

ii) 表达式

SELECT o.ID, o.Description,

CASE

WHEN (o.Status IS NULL) THEN "new"

WHEN (o.Status = 1) THEN "confirmed"

WHEN (o.Status = 3) THEN "in production"

WHEN (o.Status = 4) THEN "ready"

WHEN (o.Status = 5) THEN "shipped"

ELSE "unknown status """ || o.Status || """"

END

FROM Orders o;

12. IIF (<search_condition>, <value1>, <value2>)

说明:表达式为真,返回value1否则返回value2

等价于以下语句:

CASE

WHEN <search_condition> THEN <value1>

ELSE <value2>

END

13. NULLIF(V1,V2)

说明:如果V1=V2,返回NULL,否则返回V1

等价于以下语句:

CASE WHEN V1 = V2 THEN NULL ELSE V1 END

举例

UPDATE PRODUCTS

SET STOCK = NULLIF(STOCK,0)

14. COALESCE(V1, V2, …, Vn)

说明:如果V1为Null,返回V2,否则返回V1,

如果 n >= 3, 等于如下case语句:

CASE

WHEN V1 IS NOT NULL THEN V1

ELSE COALESCE (V2,...,Vn)

END

举例

SELECT

PROJ_NAME AS Projectname,

COALESCE(e.FULL_NAME,"[< not assigned >]") AS Employeename

FROM

PROJECT p

LEFT JOIN EMPLOYEE e

ON (e.EMP_NO = p.TEAM_LEADER);

SELECT

COALESCE(Phone,MobilePhone,"Unknown") AS "Phonenumber"

FROM Relations;