# How SQL case is used

Source: Internet
Author: User

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

`--Simple Case functionCase SexWhen' 1 'then  man ' when  ' 2 ' then  female ' else  ' then  ' man ' when sex =  ' 2 ' then  female ' 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 "second class" This result       ' First class ' in(' Second class 'ELSEEND  `

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 CountryWhenChinaThenAsiaWhenIndiaThenAsiaWhenJapanThenAsiaWhenUnited StatesThenNorth AmericaWhenCanadaThenNorth AmericaWhenMexicoThenNorth AmericaELSEOtherENDFrom Table_aGROUPByCase CountryWhenChinaThenAsiawhen  India ' then  ' Asian ' then Span style= "color: #800080;" > ' Asian ' then Span style= "color: #800080;" > ' North America ' then Span style= "color: #800080;" > ' North America ' then Span style= "color: #800080;" > ' 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;

`SELECTCaseWhen salary <= 500Then' 1 'When salary > 500and salary <= 600Then' 2 'When salary > 600and Salary <= 800Then' 3 'When salary > 800and Salary <= 1000Then' 4 'ELSENullEND Salary_class,count (*)From Table_agroup bycase when salary <= then  ' 1 ' when salary > and salary <= "then  ' 2 ' when Salary > 600 and salary <=, then  ' 3 '  and salary <= then  ' 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 when sex =  ' 1 ' else 0 end), --male population sum (case when Sex =  ' 2 ' thenpopulation else 0 --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

`' 2 ' whensalary > Endend = 1)  `

If you simply use check, as shown below

`and Salary > 1000)`

The condition of the female clerk was met, and the male clerk could not enter it.

How SQL case is used

Related Keywords:

The content source of this page is from Internet, which doesn't represent Alibaba Cloud's opinion; products and services mentioned on that page don't have any relationship with Alibaba Cloud. If the content of the page makes you feel confusing, please write us an email, we will handle the problem within 5 days after receiving your email.

If you find any instances of plagiarism from the community, please send an email to: info-contact@alibabacloud.com and provide relevant evidence. A staff member will contact you within 5 working days.

## A Free Trial That Lets You Build Big!

Start building with 50+ products and up to 12 months usage for Elastic Compute Service

• #### Sales Support

1 on 1 presale consultation

• #### After-Sales Support

24/7 Technical Support 6 Free Tickets per Quarter Faster Response

• Alibaba Cloud offers highly flexible support services tailored to meet your exact needs.