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.
Copy Code code as follows:
For example, the following SQL, you can never get the "second class" result
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.
| Chau |
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;
Two, use a SQL statement to complete the grouping of different conditions.
Have the following data
| Country (country) |
Sex (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 by 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, 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
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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, the company has a rule, the female staff must pay more than 1000 yuan. If you use check and case to behave, as shown below
Copy Code code 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
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.
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