The decode () function is one of the most powerful functions of Oracle PL/SQL and is currently available only to Oracle Corporation, where SQL implementations of other database vendors do not yet have this function.
The Decode function is one of the most powerful functions of Oracle PL/SQL, and it is currently available only to Oracle companies, and the SQL implementations of other database vendors do not yet have this function. What is the use of decode? Let's first construct an example, assuming that we want to raise wages for the smart-star staff, the standard is: wages under 8000 yuan will be added 20%, wages in the 8000 yuan plus 15%, the usual practice is to first select the record of the wage field value? Select Salary to Var-salary from employee, and then the variable var-salary is judged by a flow control statement such as If-then-else or choose case. If we use the Decode function, then we can omit these flow control statements, which can be done directly through the SQL statement. The following: Select Decode (salary-8000), 1,salary*1.15,-1,salary*1.2,salary from employee is not very concise?
DECODE syntax: DECODE (value,if1,then1,if2,then2,if3,then3,..., else), indicating that if value equals IF1, the result of the DECODE function is returned THEN1,..., Returns else if it is not equal to any of the if values. At first glance, DECODE can only do equals test, but just see, we can use some function or calculation instead of value, it is possible to make the DECODE function more than, less than or equal to the function.
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 (translated value N)
ELSE
RETURN (default value)
END IF
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 (translated value N)
ELSE
RETURN (default value)
END IF
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1, compare size
Select decode (sign (variable 1-variable 2), 1, variable 1, variable 2) from dual; --Take a smaller value
The sign () function returns 0, 1, -1
, depending on whether a value is 0, positive, or negative, for example:
Variable 1=10, variable 2=20
Sign (variable 1-variable 2) returns-1, The decode decoding result is "Variable 1", which achieves the purpose of taking the smaller value.
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2, table, view structure conversion
Existing commodity Sales table sale, table structure:
Month char (6)-- Month
Sell number (10,2)-monthly sales Amount
The existing data is:
200001,
200002 1100
200003 1200
200004 1300
200005 1400
200006
200007
200101 1100
200202 120 0
200301 1300
The data that you want to convert to the following structure:
Year char (4)--years
Month1 number (10,2)-January Sales Amount
p> month2 Number (10,2)-February Sales Amount Month3 number (10,2)-March Sales Amount
Month4 number (10,2)--April sales Amount
Month5 Number (10,2)--May Sales Amount
Month6 number (10,2)-June Sales Amount
Month7 number (10,2)--July sales Amount
Month8 Number (10,2)--August Sales Amount
Month9 number (10,2)-September Sales Amount
Month10 number (10,2)--October sales Amount
Month11 Number (10,2)--November Sales Amount
Month12 number (10,2)-December Sales amount
The SQL statements for structure conversions are:
Create or Replace view
V_sale (YEAR,MONTH1,MONTH2,MONTH3,MONTH4,MONTH5,MONTH6,MONTH7,MONTH8,MONTH9,MONTH10,MONTH11,MONTH12)
As
Select
SUBSTRB (month,1,4),
SUM (Decode (SUBSTRB (month,5,2), ' sell,0 '),
SUM (Decode (SUBSTRB (month,5,2), ' sell,0 '),
SUM (Decode (SUBSTRB (month,5,2), ' sell,0 '),
SUM (Decode (SUBSTRB (month,5,2), ' sell,0 '),
Supplement 1:
There are students score table student, now to use the Decode function to achieve the following several functions: Performance >85, show excellent, >70 show good, >60 pass;
Assuming the student number is ID and the score is score, then:
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, ' fail '))
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 decode function compares the expression with the search word, returns the result if it matches, returns the default value if it does not match, or returns a null value if no default value is defined.
Here is a simple test to illustrate the use of the Decode function:
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 system
Outln SYSTEM
Csmig SYSTEM
SCOTT SYSTEM
Eygle USERS
Dbsnmp SYSTEM
Wmsys SYSTEM 20-oct-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.