The use of decode in Oracle

Tag: Equals line tab SQL statement HTTP return hive SQL development

Original link: http://www.cnblogs.com/juddhu/archive/2012/03/07/2383101.html

Decode function usage in Oracle

Meaning explanation:
Decode (condition, value 1, return value 1, value 2, return value 2,... Value N, return 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 (translated value N)
ELSE
RETURN (default value)
END IF
Decode (field or Field operation, value 1, value 2, value 3)

The result of this function is that when the value of the field or Field operation is equal to the value 1 o'clock, the function returns the value 2, otherwise the value 3
Of course, a value of 1, a value of 2, a value of 3 can also be an expression, this function makes some SQL statements a lot simpler

How to use:
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
Then sign (variable 1-variable 2) returns-1, and thedecode decoding result is "Variable 1", which achieves the purpose of taking a smaller value.

2, this function is used in the SQL statement, the function is described as follows:

The Decode function is similar to a series of nested if-then-else statements. Base_exp and Compare1,compare2 and so on are compared in turn. If the base_exp and the compare are matched, the corresponding value of I is returned. If the base_exp does not match any of the compare values, the default is returned. Each compare value is evaluated sequentially, and if a match is found, the remaining compare values (if any) are no longer evaluated. A null base_exp is considered equivalent to the null compare value. If required, each compare value is converted to the same data type sing Woo the first compare value, which is also the type of the return value.

The decode function is very useful in practical development.

Combining the Lpad function, how to make the value of the primary key automatically add 1 and 0 in front
Select Lpad (Decode (count (record number), 0,1,max (To_number (record number) +1)), 14, ' 0 ') record number from Tetdmis

eg

Select Decode (dir,1,0,1) from A1_interval

Dir has a value of 1 to 0 and 0 to 1.

For example, I want to inquire about the number of boys and girls in a class, what is the difference?

Usually we write this:

Select COUNT (*) from table where gender = male;

Select COUNT (*) from table where sex = female;

If you want to show it together, it'll be a union, too much trouble.

With decode, you just need a word.

Select decode (Gender, male, 1,0), decode (gender, female, 1,0) from table

3,order by for a specific sort of character column

You can also use decode in order by.

Example: Table Table_subject, with subject_name columns. It is required to sort by: language, number, and order. At this point, it is very easy to use the decode to complete the requirements.

SELECT * from Table_subject ORDER by decode (subject_name, ' language ', 1, ' Math ', 2,, ' Foreign Language ', 3)

The use of decode in Oracle