sql not null constraint

Entity integrity1. Defining a primary key when building a tableCreate Table Table name (Sno int identity (+),Sname nvarchar (),--Set Primary keyPrimary Key (Sno) )2. Add a primary keyALTER TABLE table nameadd constraint pk_ table name _snoprimary KEY (ID)Referential Integrity1. Define a foreign key when building a tableCREATE TABLE table name (Sno int Identity (primary) key,CNO int not NULL,foreign Key (CNO

Tags: sql--Example 1: Create financial year table (constraint)--New table:Create Table Fiscalyears(Fiscal_Year INTEGER not NULL PRIMARY KEY,Start_date date not NULL,Constraint valid_start_dateCHECK (Year (start_date) = Fiscal_year-1and MONTH (start_date) =10And Day (start_da

1) Database Integrity ① Solid Integrity (Entity Integrity): Own A) a relationship corresponds to an entity set in the real world. --er model b) entities in the real world have some unique identity. --PRIMARY key c) The primary key is a combination of multiple attributes, and all primary properties must not be null. --Implied index ② field integrity (D

; } Else { return false; } } Else if(Baseex! =NULL!Object. ReferenceEquals (except, Baseex))//If the base class is not empty and does not equal the exception itself, proceed back to the survey to find {returnissqlreferenceconstraintexception (Baseex); } return false; } The native SQL error message is as follows: The DELE

xtype= ' PK ') Alter TABLE stuinfo Drop Constraint Pk_stuno Go--Re-add PRIMARY KEY constraint Pk_stuno ALTER TABLE stuinfo add CONSTRAINT pk_stuno PRIMARY key (Stuno) Go--Example 3 adds a unique UQ constraint (unique CONSTRAINT)--exists only A

primary key-- primary key "5. self-growth-- automatic growth of corresponding values"Example:ID int (4) Zerofill primary key auto_increment-- self-growing, starting from 0 zerofill 0 padding "6. foreign key -- constrain data for both tables when the case of two kinds of tables appears when to solving high data redundancy problems will be separate out a sheetFor example: Employee Table and departmental tablesproblem arises : when inserting employee table data, the Department of the employee tab

Tags: src technology share tab View COM object sel Bubuko scriptDelete a constraint using the SSMS database management tool1, connect the database, select data Table-"Expand Key or Constraint-" Select the constraint to delete-"Right click-" SELECT Delete. 2. In the Delete object popup box-"click OK." 3, refresh the table-"Expand the Key or

Label:SQL statement Add constraint--PRIMARY KEY constraint (Primary key constraint): The data of the primary key column is required to be unique and is not allowed to be empty. --Unique Constraint: Requires the column to be unique, allowed to be empty, but only one null valu

processing is performed in SQL, the whitespace at the end of the string is deleted. In other words, "vampire" is equivalent to "vampire", which is true for most cases (such as a string in a WHERE clause or a string in an INSERT statement), such as a query result of the following statement, as the result of querying with the user name "vampire". SELECT * from WHERE Username='vampire '; But there are exceptions, and the best exampl

Solution 1:The first idea for this question may be: Is it OK to add a unique key to the caption field? OK, let's follow this thread and create a unique index first.The code is as follows:Create unique nonclustered index UN_TEST_TBOn TEST_TB (caption)Go Index creation Okay, let's test the effect.The code is as follows:Insert into TEST_TB (caption)VALUES (NULL)GoInsert into TEST_TB (caption)VALUES (NULL)Go

SQL CHECK constraintsCHECK constraints are used to limit the range of values in a columnIf a CHECK constraint is defined on a single column, the column only allows a specific valueIf a CHECK constraint is defined on a table, the secondary constraint limits the value in a specific columnThe following

exists, exit gracefully. . } Else { //If not, and then insert a new entry $query= "INSERT into users (username, password) VALUES ('$username‘,‘$password‘)"; . . }}Use the following code to verify the logon information:PHP$username=mysql_real_escape_string($_get[' username ']);$password=mysql_real_escape_string($_get[' Password ']);$query= "SELECT username from users WHERE username= '$username' and password= '$password‘ ";$res=mysql_query($query,$database);if($res) { if(mysql_num_rows(

Tags: ar sp for on data BS AD EF nbsp--PRIMARY KEY constraint (Primary key constraint): The data of the primary key column is required to be unique and is not allowed to be empty.--Unique Constraint: Requires the column to be unique, allowed to be empty, but only one null value can occur.--Check

Definition of the primary foreign key in SQL Server:1.Create Table int Primary Key not NULL into Values (Ten, 'IT '), (+,' Finance'), (+, 'engneer ')Create Table int Primary Key not NULL int Foreign Key References Dept (DEPT_NO))The above notation, is the column level definition, defines the column field directly when defining foreign key2.create table Dept

Label:There are three ways to replace null values in query results in SQL Server. If you have one of the following tables: We can use self-connect (self-join) as a table of employee name and its corresponding manager name: SELECT E. [name], M.[name]from [ Tblemployee] as E leftJOIN[ Tblemployee] as M on= M.employeeid We can see that Todd's corresponding Manger is

Solution 1: for this problem, the first thought may be: Can I add a unique key to the caption field? Well, let's proceed with this idea and create a unique index first. Nbsp; Code: createuniquenonclusteredindexun_test_tbontest_tb (caption) go index has been created. Let's test the effect nbsp; Code: I solution 1: For this question, the first thought may be: Can I add a unique key to the caption field? Well, let's proceed with this idea and create a unique index first. The Code is as follows: C

Recently, you need to remove the primary key from a table in the ACCESS database in the program and then perform some operations. Then add the primary key. You can use this SQL statement to complete Remove primary key Alter table Table Name drop constraint primarykey The above primarykey is the name of the Integrity Constraint, And the ACCESS database uses the d

server| View The following two views are based on system table sysobjects, syscomments, and system view sysconstraints, including the table ID, table name, column ID, column name, check constraint ID, check constraint name, The check constraint status value and the contents of the check constraint, Tccview is Table-col

In fact, there is nothing to say about this problem, today's optimization encountered an SQL statement, because it is more interesting, so I intercepted, simplified the SQL statement, to show you, as shown belowDECLARE @bamboo_Code varchar (3);Set @bamboo_Code = '-01 ';SELECT DISTINCT Yarn_lotFrom Dbo.rsjob with (NOLOCK)WHERE Right (ges_no, 3) = @bamboo_CodeAs shown above, the

Is null and is not null in SQL SERVER cause index failure ?, Sqlnull In fact, there is nothing to say about this problem. I encountered an SQL statement during optimization today. Because it is interesting, I have cut and simplified the SQL statement and demonstrated it for