substr函数浅析

--substr函数浅析


{ SUBSTR| SUBSTRB| SUBSTRC| SUBSTR2| SUBSTR4}(char, position [, substring_length ])


Purpose


The SUBSTR functions return a portion of char, beginning at character position, substring_length characters long. SUBSTR calculates lengths using characters as defined by the input character set. SUBSTRB uses bytes instead of characters. SUBSTRC uses Unicode complete characters. SUBSTR2 uses UCS2 code points. SUBSTR4 uses UCS4 code points.

• If position is 0, then it is treated as 1.
• If position is positive, then Oracle Database counts from the beginning of char to find the first character.
• If position is negative, then Oracle counts backward from the end of char.
• If substring_length is omitted, then Oracle returns all characters to the end of char. If substring_length is less than 1, then Oracle returns null.

char can be any of the data types CHAR, VARCHAR2, NCHAR, NVARCHAR2, CLOB, or NCLOB. The exceptions are SUBSTRC, SUBSTR2, and SUBSTR4, which do not allow char to be a CLOB or NCLOB. Both position and substring_length must be of data type NUMBER, or any data type that can be implicitly converted to NUMBER, and must resolve to an integer. The return value is the same data type as char. Floating-point numbers passed as arguments to SUBSTR are automatically converted to integers.

--实验SCOTT@PROD2> drop table t purge;Table dropped.SCOTT@PROD2> create table t as select ename from emp;Table created.SCOTT@PROD2> select * from t;ENAME----------SMITHALLENWARDJONESMARTINBLAKECLARKSCOTTKINGTURNERADAMSJAMESFORDMILLER14 rows selected.SCOTT@PROD2> select substr(ename,0,4) from t;  --截取第一位开始到后面一共四位位置SUBSTR(ENAME,0,4----------------SMITALLEWARDJONEMARTBLAKCLARSCOTKINGTURNADAMJAMEFORDMILL14 rows selected.