SQL Server 2016 learning records - data update

Learning goals ：

1、 insert data
2、 Modifying data
3、 Delete data
4、 Processing of null value

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Learning content ：

1、 insert data

There are two ways to insert data ：

• Insert tuples
• Insert subquery results
  – Multiple tuples can be inserted at a time

Sentence format ：
INSERT
INTO < Table name > [(< Property column 1>[,< Property column 2 >…)]
VALUES (< Constant 1> [,< Constant 2>]… );
function ： Inserts a new meta group into the specified table

• INTO Clause
  Specify the table name and attribute column to insert data
  The order of attribute columns can be inconsistent with the order in the table definition
  No attribute column specified ： Indicates that a complete tuple is to be inserted , And the attributes of the attribute column are consistent with the order in the table definition
  Specify some attribute columns ： The inserted tuple takes null values on the remaining attribute columns
• VALUES Clause
  The value provided must be the same as INTO Clause match
  The number of values
  The type of value

--[ example 3.69] Put a new student tuple （ Student number ：201215128; full name ： Chen Dong ; Gender ： male ; Department ：IS; Age ：18 year ） Insert into Student In the table .
INSERT
INTO  Student (Sno,Sname,Ssex,Sdept,Sage)
VALUES ('201215128',' Chen Dong ',' male ','IS',18);

--[ example 3.70] Insert the information of student Zhang Chengmin into Student In the table .
INSERT
INTO  Student
VALUES ('201215126',' Zhang Chengmin ',' male ',18,'CS');

--[ example 3.71]  Insert a course selection record （ '200215128','1 '）.
INSERT
INTO SC(Sno,Cno)
VALUES ('201215128 ',' 1 ');
/* The relational database management system will insert the record in the new Grade The column is automatically null */

Insert subquery results
Sentence format
INSERT
INTO < Table name > [(< Property column 1> [,< Property column 2>… )]
Subquery ;
INTO Clause

• Subquery
  SELECT The clause target column must be the same as INTO Clause match
  The number of values
  The type of value

--[ example 3.72]  For every department , Ask for the average age of the students , The results are stored in the database 
-- Build table 
CREATE  TABLE  Dept_age
    ( Sdept     CHAR(15)                     /* Department name */
     Avg_age SMALLINT); 	/* The average age of students */
-- insert data 
INSERT
INTO  Dept_age(Sdept,Avg_age)
        SELECT  Sdept,AVG(Sage)
        FROM     Student
        GROUP BY Sdept;

2、 Modifying data

Sentence format
UPDATE < Table name >
SET < Name >=< expression >[,< Name >=< expression >]…
[WHERE < Conditions >];

function

• Modify the specified table to meet WHERE Tuples of clause conditions
  SET Clause gives < expression > The value of is used to replace the corresponding attribute column
  If omitted WHERE Clause , Indicates that you want to modify all tuples in the table

Three modification methods ：

• Modify the value of a tuple

--[ example 3.73]  Put students 201215121 Change your age to 22 year 
UPDATE  Student
SET Sage=22
WHERE  Sno=' 201215121 '; 

Modify the value of multiple tuples

--[ example 3.74]  Increase the age of all students 1 year . 
UPDATE Student
SET Sage= Sage+1;

Modification statement with query

--[ example 3.75]  Set the scores of all students in computer science department to zero . 
UPDATE SC
SET     Grade=0
WHERE Sno  IN
        (SELECT Sno
        FROM     Student
        WHERE  Sdept= 'CS' );

3、 Delete data

Sentence format
DELETE
FROM < Table name >
[WHERE < Conditions >];
function ： Delete the specified table WHERE Tuples of clause conditions
WHERE Clause

• Specifies the tuple to delete
  The default is to delete all tuples in the table , The definition of the table is still in the dictionary

Three ways to delete ：

- Delete the value of a tuple

--[ example 3.76]  Delete student ID as 201215128 Student records of .
DELETE
    FROM Student
    WHERE Sno= '201215128';

Delete the value of multiple tuples

--[ example 3.77]  Delete all student course selection records .
DELETE FROM SC;

Delete statement with query

--[ example 3.78]  Delete the course selection records of all students in computer science department .
DELETE FROM  SC
WHERE  Sno  IN
	(SELETE  Sno
		    FROM   Student
		    WHERE  Sdept= 'CS') ;

4、 Processing of null value

（1） Null value NULL： Generally, it means unknown information or meaningless information , This is common in relational tables

（2） stay SQL In the sentence , To judge whether the value of an attribute is null , Conditional expressions should not be used xxx=NULL, It's about using IS NULL or IS NOT NULL Look at

（3） When defining attributes , There are such regulations ：

Yes NOT NULL Constraints cannot be null
Yes UNIQUE Constraints cannot be null
The attributes of the code cannot be null

（4） Algorithm ：
The result of arithmetic operation between null value and any value is null ;
The result of the comparison operation between null value and any value is UNKNOWN;
If there is UNKNOWN, Then follow the following operation rules ：