How to use Oracle Decode functions

2. 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?
   

4. 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.
   

6. 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
   

8. 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
   

10. 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 the -1,decode decoding result as "Variable 1", which achieves the purpose of taking a smaller value.

    
    

12. 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

    200004 1300

    200005 1400

    200006

    200007.

    200101 1100

    200202

    200301 1300

    Data that you want to convert to the following structure:

    Year char (4)--years

    Month1 Number (10,2)-January Sales Amount

    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

    
    

14. 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 '),

    
    

16. 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;

    
    

18. 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.

    
    

19. 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.