Today, the above header requires a similar function, initially thought of column-to-row, but there is no good way to implement it, this function is good
The DECODE function is one of the powerful functions of oracle pl/SQL. Currently, only oracle SQL provides this function, and other database vendors do not yet implement this function. What is the purpose of DECODE?
First, let's construct an example. If we want to add a salary to a staff member of zhixing, the standard is: the salary of less than 8000 yuan will be increased by 20%; the salary of more than 8000 yuan will be increased by 15%, usually, select the salary field value in the record first? Select salary into var-salary from employee, and then use if-then-else or choose case to judge the variable var-salary.
If the DECODE function is used, we can omit these flow control statements and directly complete them through SQL statements. Select decode (sign (salary-8000), 1, salary * 1.15,-1, salary * 1.2, salary from employee is very concise?
DECODE Syntax: DECODE (value, if1, then1, if2, then2, if3, then3 ,..., else), indicating that if the value is equal to if1, the result of the DECODE function returns then1 ,..., if it is not equal to any if value, else is returned. At first glance, DECODE can only perform equals tests, but as we have seen just now, we can use some functions or computing to replace value to enable the DECODE function to have the functions greater than, less than, or equal.
DECODE(Condition, value 1, translation value 1, value 2, translation value 2,... value n, translation value n, default value)
DECODE (field, compare 1, value 1, compare 2, value 2,..., compare n, value n default value)
The function has the following meanings:
IF condition = value 1 THEN
RETURN (translation value 1)
ELSIF condition = value 2 THEN
RETURN (translation value 2)
......
ELSIF condition = value n THEN
RETURN (translation value n)
ELSE
RETURN (default)
END IF
Decode () function usage tips
· Software environment:
1. Windows NT4.0 + ORACLE 8.0.4
2. ORACLE installation path: C:/ORANT
· Usage:
1. Compare the size
Select decode (sign (variable 1-variable 2),-1, variable 1, variable 2) from dual; -- smaller value
The sign () function returns 0, 1, and-1 respectively based on a value of 0, positive, or negative.
For example:
Variable 1 = 10, variable 2 = 20
Then sign (variable 1-variable 2) returns-1, and the decode decoding result is "variable 1", achieving the goal of getting a smaller value.
2. Table and View Structure Conversion
There is a sales table sale with the following structure:
Month char (6) -- month
Billing number () -- monthly sales amount
The existing data is:
200001 1000
200002 1100
200003 1200
200004 1300
200005 1400
200006 1500
200007 1600
200101 1100
200202 1200
200301 1300
Data to be converted to the following structure:
Year char (4) -- year
Month1 number () -- sales amount in
Mon2number (February) -- sales amount in December
Month3 number (March) -- sales amount in December
Month4 number (April) -- sales amount in December
Month5 number (May) -- sales amount in December
Month6 number (June) -- sales amount in December
Month7 number (July) -- sales amount in December
Month8 number (August) -- sales amount in December
Month9 number (September) -- sales amount in December
Month10 number (October) -- sales amount in December
Month11 number (November) -- sales amount in December
Month12 number (December) -- sales amount in December
The SQL statement for structure conversion is:
Create or replace view
V_sale (year, month1, mon2, month3, month4, month5, month6, month7, month8, month9, month10, month11, month12)
As
Select
Substrb (month, 1, 4 ),
Sum (decode (substrb (month, 5, 2), '01', substring, 0 )),
Sum (decode (substrb (month, 5, 2), '02', substring, 0 )),
Sum (decode (substrb (month, 5, 2), '03', substring, 0 )),
Sum (decode (substrb (month, 5, 2), '04 ', hour, 0 )),
========================================================== ====================
Supplement 1:
Students whose student ID table is student must use the decode function to implement the following functions: score> 85, excellent;> 70: Good;> 60: pass; otherwise, fail. Suppose student is id and score is score:
Select id, decode (sign (score-85), 1, 'excellent ', 0, 'excellent',-1,
Decode (sign (score-70), 1, 'good', 0, 'good',-1,
Decode (sign (score-60), 1, 'pass', 0, 'pass',-1, 'failed ')))
From student; ========================================================== ================= Supplement 2:
The syntax structure of the Decode function is as follows:
Decode (expression, search_1, result_1)
Decode (expression, search_1, result_1, search_2, result_2)
Decode (expression, search_1, result_1, search_2, result_2,..., search_n, result_n)
Decode (expression, search_1, result_1, default)
Decode (expression, search_1, result_1, search_2, result_2, default)
Decode (expression, search_1, result_1, search_2, result_2,..., search_n, result_n, default)
The comparison expression and search word of the decode function. If the expression matches, the return result is returned. If the expression does not match, the default value is returned. If the default value is not defined, the return value is null.
The following is a simple test to Decode the function usage:
SQL> create table t as select username, default_tablespace, lock_date from dba_users;
Table created.
SQL> select * from t;
USERNAME DEFAULT_TABLESPACE LOCK_DATE
---------------------------------------------------------------------
SYS SYSTEM
SYSTEM
OUTLN SYSTEM
CSMIG SYSTEM
SCOTT SYSTEM
EYGLE USERS
DBSNMP SYSTEM
Wmsys system 20--0-04
8 rows selected.
SQL> select username, decode (lock_date, null, 'unlocked', 'locked') status from t;
USERNAME STATUS
--------------------------------------
SYS unlocked
SYSTEM unlocked
OUTLN unlocked
CSMIG unlocked
SCOTT unlocked
EYGLE unlocked
DBSNMP unlocked
WMSYS locked
8 rows selected.
SQL> select username, decode (lock_date, null, 'unlocked') status from t;
USERNAME STATUS
--------------------------------------
SYS unlocked
SYSTEM unlocked
OUTLN unlocked
CSMIG unlocked
SCOTT unlocked
EYGLE unlocked
DBSNMP unlocked
WMSYS
8 rows selected.