MySQL operation query results case-then-else End usage example

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)
USA 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.
Continent 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
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 ' 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,
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 to complete the different conditions of the grouping.

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
Country men and women
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 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

CONSTRAINT check_salary Check
(Sex = ' 2 ' and salary > 1000)

The female clerk's condition was met, and the male clerk could not enter the

Four, according to the conditions have 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
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 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-sheet three
B 2 John Doe
C 3 Harry


Assuming that there is data, you need to exchange the primary key A and B with each other. Use the case function to implement the 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 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
Study 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 Archaeology 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) 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 who have chosen only one course
SELECT std_id, MAX (class_id) as Main_class
From StudentClass
GROUP by std_id
Having COUNT (*) = 1;

Execution Results 1

std_id Main_class
------   ----------
300 4
400 5
500 6

Condition 2

-Condition 2: Students who choose multiple courses
SELECT std_id, class_id as Main_class
From StudentClass
WHERE main_class_flg = ' Y ';

Execution Results 2

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

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 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 NULL and then ' wrong '
END

In this statement, when NULL is always returned to unknown, there will never be a wrong case. Because this sentence can be replaced when col_1 = NULL, this is a wrong use, this time we should choose to use when Col_1 is NULL.

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Example 1:

Using this query, you get the value of Ifavoriteid,ifavusertype, Cuser,iarticleid,dfavoritetime five fields:

SELECT Ifavoriteid,
case where Ifavusertype = 0 Then ' news manager '
When ifavusertype = 1 Then ' merchant '
When ifavusertype = 2 Then ' member '
When ifavusertype = 3 Then ' unregistered '
When ifavusertype = 4 Then ' anonymous '
END as Ifavusertype, Cuser, Iarticleid,
CONVERT (nvarchar, dfavoritetime, 111) as Dfavoritetime from Dig_favorite

Example 2:

SELECT case ' MemberType ' =1
Then ' players '
ELSE ' instructor '
END from ' Tab_sign_member '
WHERE 1

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Here's an example of how to use the three case-when statements in MySQL for your reference, if you're interested in using the case-when statement in MySQL, you might want to look at it.

1.

1. Select Name,
2. Case
3. When Birthday< ' 1981 ' then ' old '
4. When birthday> ' 1988 ' Then ' Yong '
5. Else ' OK ' END Yorn
6. from Lee;

2.

1. Select NAME,
2. Case Name
3. When the ' Sam ' Then ' Yong '
4. When the ' Lee ' then ' Handsome '
5. Else ' good ' end
6. from Lee;

Of course, the case-when statement can also be composited

3.

1. Select Name,birthday,
2. Case
3. When birthday> ' 1983 ' Then ' Yong '
4. When name=' Lee ' then ' Handsome '
5. Else ' Just so ' end
6. from Lee;

The above is an introduction to the use examples of case-when statements in MySQL.

MySQL operation query results case-then-else End usage example