How to use case in SQL
Case has two formats. Simple case function and case search function.
Copy Code code as follows:
--Simple Case function
Case Sex
When ' 1 ' THEN ' male '
When ' 2 ' THEN ' woman '
Else ' other ' end
--case search function
case when sex = ' 1 ' THEN ' man '
When sex = ' 2 ' THEN ' female '
Else ' other ' end
In both of these ways, you can achieve the same functionality. The simple case function is relatively concise, but there are some limitations to the function, such as writing a judgment, compared with the search function.
There is also a problem to note that the case function returns only the first qualifying value, and the remainder of the case is automatically ignored.
For example, the following SQL, you can never get the "second class" result
Copy Code code as follows:
Case when col_1 in (' A ', ' B ') THEN ' first class '
When Col_1 in (' a ') THEN ' Class II '
Else ' other ' end
Let's take a look at what we can do with the case function.
First, the known data is grouped and analyzed in a different way.
Have the following data: (in order to see more clearly, I did not use the country code, but directly using 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. Should get the following result.
Continent population
Asia 1100
North America 250
Other 700
What would you do to solve the problem? Creating a view with state code is a solution, but it is difficult to dynamically change the way statistics are used.
If you use the case function, the SQL code is as follows:
Copy Code code 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 ' Mexican ' THEN ' North America '
Else ' other ' end
From Table_a
GROUP by 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 ' Mexican ' THEN ' North America '
Else ' other ' end;
Similarly, we can use this method to determine the level of wages, and statistics of the number of each level. The SQL code is as follows;
Copy Code code as follows:
SELECT
Case when salary <= THEN ' 1 '
When salary > Salary <= THEN ' 2 '
When salary > Salary <= THEN ' 3 '
When salary > Salary <= 1000 THEN ' 4 '
ELSE NULL End Salary_class,
COUNT (*)
From Table_a
GROUP by
Case when salary <= THEN ' 1 '
When salary > Salary <= THEN ' 2 '
When salary > Salary <= THEN ' 3 '
When salary > Salary <= 1000 THEN ' 4 '
ELSE NULL end;
Second, use a SQL statement to complete the grouping of different conditions.
Have the following data
National (country) gender (sex) population (population)
China 1 340
China 2 260
The United States 1 45
The United States 2 55
Canada 1 51
Canada 2 49
United Kingdom 1 40
United Kingdom 2 60
Grouped by country and gender, the results are as follows
National men and women
China 340 260
The United States 45 55
Canada 51 49
United Kingdom 40 60
In general, the Union can also be implemented with a statement to query. But that increases consumption (two select parts), and the SQL statement is longer.
Here is an example of using the case function to complete this function
Copy Code code as follows:
SELECT Country,
SUM (case when sex = ' 1 ' THEN
Population ELSE 0 end),--Male population
SUM (case when 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 two-dimensional table, fully show the powerful case function.
Third, use the case function in check.
Using the case function in check is a very good solution in many cases. There may be a lot of people who don't check at all, so I suggest you try using check in SQL after looking at the example below.
Now let's take an example
Company A, this company has a rule, the female staff's salary must be higher than the block. If you use check and case to behave, as shown below
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CONSTRAINT check_salary Check
(Case when sex = ' 2 ')
THEN Case when salary > 1000
THEN 1 ELSE 0 End
ELSE 1 end = 1)
If you simply use check, as shown below
Copy Code code as follows:
CONSTRAINT check_salary Check
(Sex = ' 2 ' and salary > 1000)
The condition of the female staff was met, and the male staff could not enter it.
Four, according to the conditional have selected update.
example, there are the following update conditions
Staff with a salary reduction
Wages in between to the staff, the increase in wages%
It is easy to consider the option of executing the two UPDATE statement as follows
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--condition
UPDATE Personnel
SET salary = salary * 0.9
WHERE Salary >= 5000;
--condition
UPDATE Personnel
SET Salary = salary * 1.15
WHERE Salary >= and salary < 4600;
But it's not as simple as it might seem, assuming there's a personal wage block. First of all, according to the conditions, wages reduced to become wages. Then run the second SQL, because this person's salary is within the range, need to increase%, the last person's salary is not reduced, but increased. If it were to be done in reverse, then the person who paid would turn into a reduction in wages. Regardless of how absurd this regulation is, if you want an SQL statement to implement this function, we need to use the case function. The code is as follows:
Copy Code code as follows:
UPDATE Personnel
SET salary = case when salary >= 5000
THEN Salary * 0.9
When salary >= and 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 this is not the case, the wages of those who do not meet these two conditions will be written null, and that would be a bad thing. The default value for the else part in the case function is null, and this is where you need to be aware.
This method can also be used in many places, such as changing the primary key such dirty.
In general, to two data primary Key,a and B exchange, the need for temporary storage, copy, read back the data of the three processes, if the use of case functions, everything becomes much simpler.
P_key Col_1 col_2
A 1 sheets of three
B 2 Dick
C 3 Harry
Suppose you have data, you need to exchange primary keys A and B. With the case function, the code is as follows
Copy Code code as follows:
UPDATE sometable
SET P_key = case when P_key = ' a '
THEN ' B '
When p_key = ' B '
THEN ' a '
ELSE P_key End
WHERE P_key in (' A ', ' B ');
The same can also be exchanged for two unique key. It should be noted that if there is a need to exchange the primary key, 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 consistent check.
The case function is different from the Decode function. In the case function, you can use Between,like,is null,in,exists and so on. For example, the use of in,exists, can be subqueries, so as to achieve more functionality.
Here is an example to illustrate that there are two tables, tbl_a,tbl_b, and KeyCol columns in all two tables. Now we are comparing two tables, the KeyCol column in Tbl_a can be found in the Tbl_b keycol column data, return the result ' matched ', if not found, return the result ' unmatched '.
To implement this feature, you can use the following two statements
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--When using in
SELECT KeyCol,
Case when KeyCol in (SELECT keycol from Tbl_b)
THEN ' matched '
ELSE ' unmatched ' end Label
From Tbl_a;
--When using the Exists
SELECT KeyCol,
Case when 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 in and not EXISTS, but you should pay attention to null at this time.
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 History N
200 2 History N
200 3 Archaeological Y
200 4 Computer N
300 4 Computer N
400 5 Chemical N
500 6 Mathematics N
Some students choose to take several courses at the same time (100,200) and some students only choose one course (300,400,500). Students taking multiple courses are asked to choose a course as their major, majoring in flag to write Y. Students who choose only one course, majoring in flag n (in fact, if you write Y, there is no trouble below, in order to give an example, also please include).
Now we're going to query this table in terms of the following two conditions
The person who took only one course returned the ID of the course.
People who take multiple courses, return the selected main course ID
The simple idea is to execute two different SQL statements for querying.
Conditions
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--Condition: A student who has chosen only one course
SELECT std_id, MAX (class_id) as Main_class
From StudentClass
GROUP by std_id
Having COUNT (*) = 1;
Execution results
std_id Main_class
------ ----------
300 4
400 5
500 6
Conditions
Copy Code code as follows:
--Conditions: Students who choose multiple subjects
SELECT std_id, class_id as Main_class
From StudentClass
WHERE main_class_flg = ' Y ';
Execution results
std_id Main_class
------ ----------
100 1
200 3
If you use the case function, we can solve the problem with just one SQL statement, as shown below
Copy Code code as follows:
SELECT std_id,
Case when COUNT (*) = 1-The situation of a student who chooses only one course
THEN MAX (class_id)
ELSE MAX (case when 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
------ ----------
100 1
200 3
300 4
400 5
500 6
By nesting the case function in the case function and using the case function in the aggregate function, we can solve this problem easily. Using the case function gives us greater freedom.
Finally, remind the novice using the case function to be careful not to make the following error
Copy Code code as follows:
Case Col_1
When 1 THEN ' right '
When NULL THEN ' wrong '
End
When NULL is always returned to unknown in this statement, wrong is never present. Because this sentence can be replaced by when col_1 = NULL, this is a wrong use, this time we should choose to use when Col_1 is NULL.