Database hyperphone (I)

1. How to modify a data table

Use alter table command
(1)add

alter table < Table name >
add < Column definition >|< Integrity constraint definition >

Example ：

（1） Add column : 
alter table S 
add sn int(6)


（2） Add default value constraint 
alter table S
add constraint DF_S_T default(2) for time

(3） Add primary key constraint 
alter table S 
add constraint PK_S_Id primary key(id)

(4) Add a unique key constraint 
alter table S
add constraint UQ_S_Name unique(id)

(5) Add foreign key constraints 
alter table SC 
add constraint FK_SC_S foreign key(sno) references S(sno)

6） increase check constraint 
alter table SC 
add constraint Score_Chk  check(score between 0 and 100)

Be careful ：

• New columns added in this way are automatically filled NULL value , Therefore, you cannot specify NOT NULL constraint .

(2)alter

alter table < Table name >
alter column< Name >< data type >[null|not null]

Example ：

（1） Widen the column to 12 character 
alter table S
alter column sn nvarchar(12)
（2） Change data type 
alter table TbStudent 
alter column stuGender nchar(1)
（3） Modify constraint type 
alter table TbStudent alter column stuPhone char(11) not null

Be careful ：

• The column name cannot be changed ;

• You cannot change the definition of a column with a null value to NOT NULL constraint ;

• If there is data in the column , Then the width of the column cannot be reduced , Also cannot change its data type ;

• It can only be modified NULL/NOT NULL constraint , Other types of constraints must be deleted before modification , Then add the modified constraint definition again .

(3)drop

alter table < Table name >
dorp aonstraint < Constraint name >

alter table < Table name >
drop column < Name >

 Example ：
（1） Delete constraints 
alter table S 
drop constraint UQ_S_Name
（2） Delete column 
alter table S 
drop column dept

2. distinguish count、count(*)

count(*)： Count the number of tuples in the table . You can't go heavy , Calculation NULL value

count([distinct] < Name >)： Count the number of column values . You can get rid of it , Don't count NULL

Be careful ：

• In library functions , except COUNT（*） Outside , Other functions are ignored in the calculation NULL value .

（1） Ask how many departments there are in the school .
SELECT COUNT(DISTINCT Dept) AS DeptNum  FROM S


（2） Count the number of column values with achievements .
SELECT COUNT (Score)   FROM SC
 The result in the above example is 0 Students are also counted , No results （ It is null ） Do not calculate .

（3) Find out that the selected courses of the computer department are less than 3 The name of the student of the door 、 The number of elective courses , Including students who don't choose courses . It is required to sort in descending order according to the number of courses selected , If the number of courses selected is the same , Sort by name in ascending order 
select sn,count(cno)  The number of elective courses  
from S left outer join SC on S.sno = SC.sno
where dept=" Department of Computer Science "
group by S.sno 
having count(cno)<3
order by count(cno) desc sn asc
（4）S The male students in the table are from each age group （ exceed 50 people ） How many people are there , Sort in ascending order by the number of people , The same number is in descending order of age 
select <font color='red'>age </font>,count(*)  The number of   # Notice the age Don't forget 
from S
where sex=' male '
group by age
having count( * )> 50
order by count(*),age desc

The first （3） Questions cannot be used count( * ), Among the students who did not choose courses , Use count(*) Still calculate the number of courses , While using count（cno）, Directly count the number of course numbers of each group , The number of courses selected can be calculated correctly
Also note ,count and group by Continuous time ,count The number of statistics is group by After grouping, the number of data in each group , Not the number of groups .

3.SQL The order in which statements are executed

stay SQLServer in

The picture is drawn for the online boss

4. distinguish where、having And clear group by It's using

• GROUP BY Clause is grouped by column Must be a column name that exists in the table , Out of commission AS Column alias assigned by clause .
• Appoint GROUP BY when , In the selection list All columns in any non aggregate expression Should be included in
• WHERE Clause used Screening FROM The row data generated by the data source specified in clause .
• GROUP BY Clauses are used to justify WHERE After Clause filtering Results data for Grouping .
• HAVING Clause is used for After grouping And then filter the statistical results .
• HAVING Clause can use Three elements ： constant , Aggregate functions ,GROUP BY The column name specified in clause ( Juejian )

1. Query the number of students in each department of computer department and information management department .

 The first one is ：#( correct ）
SELECT Sdept, COUNT(*)  FROM Student
  GROUP BY Sdept
  HAVING Sdept in ( ' Department of Computer Science ', ' Information management department ')
 The second kind ：#( correct ）
SELECT sdept, COUNT (*)  FROM Student
  WHERE Sdept in ( ' Department of Computer Science ', ' Information management department ')
  GROUP BY Sdept

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2. Query the age of each department is less than or equal to 20 The number of students .#( correct ）
 SELECT sdept,COUNT (*) FROM Student
 WHERE Sage <= 20
 GROUP BY Sdept
SELECT Sdept, COUNT(*) FROM Student# ( error ）
  GROUP BY Sdept
  HAVING Sage <= 20

The second reason for the mistake is Sage Not in the group by It appears that

Be careful ：

1. Filter criteria that can be applied before the grouping operation , stay WHERE Clause is more effective .
2. stay HAVING The filter conditions specified in clause should be those that must be applied after the grouping operation .
3. Put all the filter conditions that should be carried out before grouping in WHERE Clause instead of HAVING clause .

5. Commonly used data manipulation language and data definition language syntax

Modify data sheet
Alter table < Table name >
Add < Column definition > |< Integrity constraints >

Alter table < Table name >
Alter colum < Name >< data type > [null | not null]

Alter table < Table name >
drop constraint < Constraint name >|colum< Name >

Delete base table
Drop table < Table name >

Add data
Insert into < Table name > [(< Name 1>[,< Name 2>])]
Values (< value >) | Subquery

Modifying data
Update < Table name >
Set < Name >=< expression >[,< Name >=< expression >]
[where < Conditions >]

Delete data
Delete
From < Table name >
[where < Conditions >]

Create view
Create view < View name >[( View column name )[, ( View column name )]
As Query statement

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