Case has two formats. Simple case function and case search function.
--Simple Case function
Case Sex
When ' 1 ' Then ' men '
When ' 2 ' then ' women '
Else ' other ' END
--case search function
case If sex = ' 1 ' Then ' man '
When sex = ' 2 ' Then ' women '
Else ' other ' END
In both of these ways, the same functionality can be achieved. The simple case function is relatively concise, but there are some limitations in function, such as write-judgement, compared to the search function.
There is also a problem to be aware that the case function returns only the first qualifying value, and the remaining case section is automatically ignored.
--for example, the following SQL, you can never get the result of "type two"
Case is col_1 in (' A ', ' B ') then ' first Class '
When Col_1 in (' a ') then ' Second class '
Else ' other ' END
Let's take a look at what you can do with the case function.
One, the known data in a different way to group, analysis.
There is the following data: (in order to see more clearly, I did not use the country code, but directly with the country name as primary Key)
| Country (country) |
Population (population) |
| China |
600 |
| United States |
100 |
| Canada |
100 |
| United Kingdom |
200 |
| France |
300 |
| Japan |
250 |
| Germany |
200 |
| Mexico |
50 |
| India |
250 |
According to the population data of this country, the population of Asia and North America is counted. The following result should be obtained.
| Chau |
Population |
| Asia |
1100 |
| North america |
250 |
| Other |
700 |
What would you do to solve this problem? Creating a view with a continent code is a workaround, but it is difficult to dynamically change the way statistics are used.
If you use the case function, the SQL code is as follows:
SELECT SUM (population),
Case Country
When ' China ' then ' Asia '
When ' India ' then ' Asia '
When ' Japan ' then ' Asia '
When ' America ' then ' North America '
When ' Canada ' then ' North America '
When ' Mexico ' then ' North America '
Else ' other ' END
From table_a
Case Country
When ' China ' then ' Asia '
When ' India ' then ' Asia '
When ' Japan ' then ' Asia '
When ' America ' then ' North America '
When ' Canada ' then ' North America '
When ' Mexico ' then ' North America '
Else ' other ' END;
Similarly, we can use this method to judge the salary level, and to count the number of each level. The SQL code is as follows;
SELECT
Case when salary <= ' 1 '
When salary > Salary <= 2 '
When salary > Salary <= 3 '
When salary > Salary <= "4"
ELSE NULL END Salary_class,--alias name
COUNT (*)
From table_a
GROUP by
Case when salary <= ' 1 '
When salary > Salary <= 2 '
When salary > Salary <= 3 '
When salary > Salary <= "4"
ELSE NULL END;
Two, a SQL statement is used to complete the grouping of different conditions.
Have the following data
| Country (country) |
Gender (Sex) |
Population (population) |
| China |
1 |
340 |
| China |
2 |
260 |
| United States |
1 |
45 |
| United States |
2 |
55 |
| Canada |
1 |
51 |
| Canada |
2 |
49 |
| United Kingdom |
1 |
40 |
| United Kingdom |
2 |
60 |
Grouped according to country and gender, the results are as follows
| Countries |
Man |
Woman |
| China |
340 |
260 |
| United States |
45 |
55 |
| Canada |
51 |
49 |
| United Kingdom |
40 |
60 |
In general, a Union can also be used to implement a query with a single statement. But that increases the consumption (two select parts), and the SQL statement is longer.
Here is an example of using the case function to accomplish this function
SELECT Country,
SUM (case if sex = ' 1 ' then population ELSE 0 END), --male population
SUM (case if sex = ' 2 ' then population ELSE 0 END) --female population
From table_a GROUP by country;
In this way, we use Select to complete the output form of the two-dimensional table, which fully shows the strong case function.
third, use the case function in check.
Using the case function in check is a very good workaround in many cases. There may be a lot of people who don't have check at all, so I suggest you try using check in SQL after reading the following example.
Let's give an example.
Company A, the company has a rule that female employees must pay more than 1000 yuan. If you use check and case to behave as follows
CONSTRAINT check_salary Check (case if sex = ' 2 ' then case is salary > then 1 else 0 END Else 1 END = 1)
If you simply use check, as shown below
CONSTRAINT check_salary check (sex = ' 2 ' and salary > 1000)
The condition of the female clerk was met, and the male clerk could not enter it.
four, according to the conditions of the selected update.
example, there are the following update conditions
- Employees with a salary of more than 5000 are reduced by 10%
- Salary increased by 15% for employees between 2000 and 4600
It is easy to consider the option to execute two UPDATE statements as follows
--Condition 1
UPDATE personnel SET salary = salary * 0.9
WHERE Salary >= 5000;
--Condition 2
UPDATE personnel SET salary = salary * 1.15
WHERE Salary >= and salary < 4600;
But it's not as simple as it is supposed to be, assuming a personal salary of 5000 bucks. First of all, according to condition 1, wages are reduced by 10% to 4500 of wages. Next run the second SQL, because this person's salary is 4500 in the range of 2000 to 4600, need to increase 15%, and finally this person's salary result is 5175, not only not reduced, but increased. If the reverse is done, then the wage of 4600 will turn into a wage reduction. No matter how absurd the rules are, if you want an SQL statement to implement this function, we need to use the case function. The code is as follows:
UPDATE Personnel
Case when salary >= salary * 0.9
When salary >=, salary < 4600 then salary * 1.15
ELSE salary END;
It is important to note here that the last line of else salary is required, and if there is no such line, the wages of those who do not meet these two conditions will be written as NULL, and that will be a big bad thing. The default value of the else part in the case function is null, which is something to be aware of.
This method can also be used in many places, such as changing the primary key dirty.
In general, to the two data primary key,a and B exchange, need to be temporarily stored, copied, read back the data of the three processes, if you use the case function, everything becomes much simpler.
| P_key |
Col_1 |
Col_2 |
| A |
1 |
Tom |
| B |
2 |
John doe |
| C |
3 |
Harry |
Assuming that you have the data, you need to put the primary keyaAndbExchange with each other. Use the case function to implement the code as follows
UPDATE sometable
SET P_key = P_key = ' A ' then ' B '
When p_key = ' B ' then ' a ' ELSE p_key END
WHERE P_key in (' A ', ' B ');
The same can be exchanged for two unique keys. It should be noted that if there is a need to exchange the primary key occurs, most of the original design of the table is not in place, it is recommended to check the design of the table is appropriate.
Five, two table data is checked for consistency.
The case function differs from the DECODE function. In the case function, you can use Between,like,is null,in,exists and so on. For example, using in,exists, you can make subqueries to achieve more functionality.
The following example shows that there are two tables, tbl_a,tbl_b, and two tables with KeyCol columns. Now we compare two tables, the data in the KeyCol column in the tbl_a can be found in the data of the KeyCol column in Tbl_b, return the result ' matched ', and if not found, return the result ' unmatched '.
To implement this function, you can use the following two statements
--When using in
SELECT KeyCol,
Case if KeyCol in (SELECT keycol from tbl_b) then ' matched '
ELSE ' unmatched ' END Label
From Tbl_a;
-When using the Exists
SELECT KeyCol,
Case if EXISTS (SELECT * from Tbl_b WHERE tbl_a.keycol = tbl_b.keycol) then ' matched ' ELSE ' unmatched ' END Label
From Tbl_a;
The results of using in and exists are the same. You can also use not and not EXISTS, but be aware of the null situation at this time.
Six, use the aggregate function in the case function
Suppose you have one of the following tables
| School Number (STD_ID) |
Course ID (class_id) |
Course Name (class_name) |
Major in Flag (MAIN_CLASS_FLG) |
| 100 |
1 |
Economics |
Y |
| 100 |
2 |
Historical studies |
N |
| 200 |
2 |
Historical studies |
N |
| 200 |
3 |
Archaeology |
Y |
| 200 |
4 |
Computer |
N |
| 300 |
4 |
Computer |
N |
| 400 |
5 |
Chemical |
N |
| 500 |
6 |
Mathematical |
N |
Some students choose to take several courses at the same time (100,200) Some students choose only one course (300,400,500). Students taking multiple courses are asked to choose a course as their major, with the major flag written in Y. Students who choose only one course, majoring in flag n (in fact, if written in Y, will not have the following trouble, in order to give examples, please include more).
Now we're going to query this table according to the following two conditions
- People who take only one course, return the ID of that course
- For those who take multiple courses, return to the selected master course ID
The simple idea is to execute two different SQL statements to query.
Condition 1
--Condition 1: Students selected only one course select std_id, MAX (class_id) as Main_class from studentclass GROUP by std_id have COUNT (*) = 1;
Execution Results 1
std_id Main_class
4
5
6
Condition 2
--Condition 2: Select std_id for multiple courses , class_id as Main_class from studentclass WHERE main_class_flg = ' Y ';
Execution Results 2
std_id Main_class
1
3
If you use the case function, we can solve the problem with just one SQL statement, as shown below
SELECT std_id,
When COUNT (*) = 1 --The case of a student who chooses only one course then MAX (class_id)
MAX (case if main_class_flg = ' Y ' then class_id ELSE NULL END )
END as Main_class
From StudentClass GROUP by std_id;
Run results
std_id Main_class
1
3
4
5
6
By nesting the case function in the case function and using the case function in the aggregate function, we can easily solve the problem. Using the case function gives us a greater degree of freedom.
Finally, a reminder to use the CASE function Novice Note do not make the following error
Case Col_1
When 1 then ' right '
When NULLand then ' wrong '
END
In this statement, when NULL is always returned to unknown, there will never be a wrong case. Because the actual expression of this sentence means
When col_1 = null, this is a wrong usage, and this time we should choose when Col_1 is null.
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Use of SELECT case
The maximum benefit of combining select with case is two points, one is the flexible organization format when displaying query results, and the other is the effective avoidance of multiple access to the same table or several tables. Here is a simple example to illustrate. For example table students (ID, name, birthday, sex, grade), the number of boys and girls required by each grade statistics, the results of the table head for, grade, the number of boys, the number of girls. If you do not need a Select case, in order to show the number of men and women, statistically very troublesome, first determine the grade information, and then according to the grade of boys and girls, and very prone to error. Use the Select Case when you have the following wording:
SELECT grade, COUNT (case if sex = 1 then 1/*sex 1 for boys, 2 girls */
ELSE NULL
END) Number of boys,
COUNT (case if sex = 2 then 1
ELSE NULL
END) Number of girls
From students GROUP by grade;
Use of the ORACLE case when and SELECT case
