Table of contents
1. Use decode to judge the string or value
2. Use the decode joint sign() function to compare the size
3. Use the decode function to segment
4. Use decode to realize row and column conversion of tables or views
5. Use the decode function to search for strings using expressions
DECODE is an exclusive function provided by Oracle Corporation, and it is a very powerful function. It's not standard for SQL, but it's very useful for performance.
Common scenarios for the decode function:
1. Use decode to judge the string or value
1.1DECODE(value,if1,then1,if2,then2,if3,then3,...,else)
The meaning of sql is:
IF 条件=值1 THEN
RETURN(value 1)
ELS IF 条件=值2 THEN
RETURN(value 2)
ELS IF 条件=值n THEN
RETURN(value 3)
ELSE
RETURN(default)
END IFsql code test
When the value of the field sex is 1, the value is male, when the value is 0, it is female, otherwise other values are other;
(Note: the following output results must all be of the same type)
select decode(sex,1,'boy',0,'girl','other')
select decode(sex,'boy',1,'girl',0,-1)
2. Use the decode joint sign() function to compare the size
decode(sign(var1-var2),-1,var1,var2)
Introduce the usage of sign() function
The sign() function returns 0, 1, -1 according to whether the value in the parentheses is 0, positive or negative
sql test
select decode(sign(100-90),1,'100',-1,'90','equal') -- the output result is 100
3. Use the decode function to segment
A score greater than 90 is considered excellent, a score between 80-90 is considered good, and a score below 80 is considered to be improved;
sql test
select decode(sign(score-90),1,'excellent',0,'excellent',-1,decode(sign(score-80),1,'good',0,'good',-1,' To be improved'))
4. Use decode to realize row and column conversion of tables or views
sql test
SELECT
SUM(DECODE(ENAME,'SMITH',SAL,0)) SMITH,
SUM(DECODE(ENAME,'ALL',SAL,0)) ALL,
SUM(DECODE(ENAME,'WARD',SAL,0)) WARD,
SUM(DECODE(ENAME,'JONES',SAL,0)) JONES,
SUM ( DECODE ( NAME , ' MARTIN ' , SAL , 0 )) MARTIN FROM EMP
The output is as follows
SMITH ALLEN WARD JONES MARTIN
800 1600 1250 2975 1250
5. Use the decode function to search for strings using expressions
decode (expression, search_1, result_1, search_2, result_2, ...., search_n, result_n, default)
The decode function compares the expression and the search word, and returns a result of 1 if they match, a default value of 0 if they do not match, or a null value if no default value is defined.
sql test
SELECT ENAME,SAL,DECODE(INSTR(ENAME, 'S'),0,'does not contain s','contains s') AS INFO FROM EMP
output result
SMITH 800 contains s
ALLEN 1600 does not contain s
WARD 1250 does not contain
JONES 2975 contains s
MARTIN 1250 does not contain s
BLAKE 2850 does not contain s
CLARK 2450 does not contain s
SCOTT 3000 contains s
KING 5000 does not contain s
TURNER 1500 does not contain s
Adams 1100 contains s