case in syntax SQL then else end usage-resolves a field to take a different value based on the condition

Case has two formats. Simple case function and case search function.

--Simple case function when '         1 ' then ' Male ' when         ' 2 ' then ' women ' else ' other ' end--case search function case when 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 a "second class" When the case is col_1 in (' A ', ' B ') then ' first Class ' When         col_1 in (' a ') then       ' second ' else ' other ' END

Let's look at what we can do with the case function.  

One, the known data is grouped in another way, analyzed.  

has the following data: (in order to see more clearly, I do 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 ' us ' then ' North America ' when                ' Canada ' Then ' North America ' when                ' Mexico ' then  ' North America '        Else ' other ' endfrom    Table_agroup by case country when '                China ' then ' Asia ' when '                India ' then     ' Asia ' when                ' Japan ' then     ' Asia ' C22/>when ' Us ' 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 <= and  "2" when             salary > Salary <=  3 ' when             salary > Salary <= and ' 4 '        ELSE NULL END Salar  Y_class,        COUNT (*) from Table_agroup by case when        salary <= and ' 1 ' when             salary > Salary  <=  ' 2 ' when             salary > Salary <= and the  ' 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
US	1	45
US	2	55
Canada	1	51
Canada	2	49
UK	1	40
UK	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_agroup 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

1. Employees with a salary of more than 5000 are reduced by 10%
2. 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 1UPDATE personnelset salary = salary * 0.9WHERE salary >= 5000;--conditions 2UPDATE personnelset salary = salary * 1.15WHERE 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 Personnelset salary = case when salary >= and            salary * 0.9WHEN salary >= and salary < 4600 Then salary * 1.15ELSE 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 the data is the same, you need to exchange the primary key a with b each other. Use the case function to implement the code as follows

UPDATE Sometableset P_key = case if P_key = ' A ' Then ' B ' when p_key = ' B ' and ' A ' ELSE p_key endwhere 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 when keycol in (select KeyCol from tbl_b) Then ' matched ' ELSE ' unmatched ' END labelfrom tbl_a; --When using EXISTS, select Keycol,case when EXISTS (SELECT * from Tbl_bwhere tbl_a.keycol = tbl_b.keycol) Then ' matched ' ELSE ' Un Matched ' END labelfrom 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

1. People who take only one course, return the ID of that course
2. 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: Only one course of students selected std_id, MAX (class_id) as Main_classfrom Studentclassgroup by Std_idhaving COUNT (*) = 1;

Execution Results 1

std_id   main_class------   ----------      4400      5500      6

Condition 2

-Condition 2: Select std_id for multiple courses, class_id as Main_classfrom studentclasswhere main_class_flg = ' Y ';

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 case of a student who chooses only one course then Max (class_id) ELSE max (cases when main_class_flg = ' Y ' Then Class_idelse NULL end) End as Main_classfrom studentclassgroup by std_id;

Run results

std_id   main_class------   ----------      3300      4400      5500      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_1when 1 then   ' right ' when NULL 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.

case in syntax SQL then else end usage-resolves a field to take a different value based on the condition