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UseText_dataGo if exists(Select * fromsysobjectswhereName='TextInfo')--detecting the presence of a textinfo table Drop TableTextInfo--Delete Table TextInfo Create TableTextInfo--Create a table TextInfo (Tidint Identity(1,1)Primary Key not NULL,--primary key, identity column, not emptyTinfonvarchar( $) not NULL--cannot be empty)Add constraintGrammar: ALTER TABLE table name Add
in the Department table), the insert succeeds:2.7 Non-null constraintsA non-null constraint (NOT NULL), which can be understood by a name, is a non-null constrained column that must not be empty when inserting a value.Violations of non-
Label:1.NULL meaning is the missing value (missing value). 2. Three value logic (Three-valued-logic:true,false,unknown). There are three logical predicates in SQL: Ture,false,unknown. In most programming language only True and false in SQL , and all of the existence of unknown is related to null. For example, make the
Unique constraints of the SQLUnique identification of unique constraints per record in a database table.Both the uniqueness and primary constraints provide the guarantee of a single column or set of columns.A PRIMARY KEY constraint automatically has a unique constraint that defines it.Please note that you can have many unique restrictions on each table, but only one primary KEY
The table structure is created like the following code Copy Code code as follows: CREATE TABLE TEST_TB ( TestID int NOT null identity (1,1) primary key, Caption nvarchar (MB) null ); Go 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. Copy Code code as
Label:1,Primary KeyConstraints There are five types of constraints in SQL Server, Primary key constraints, Foreign key constraints, Unique constraints, Default constraints, and Check constraints. There is often a combination of one or more columns in a table whose values uniquely identify each row in the table. Such a column or columns becomes the primary key (PrimaryKey) of the table. A table can have only one primary key, and a column in a PRIMARY K
In SQL database, what should you do if you need to delete a table constraint? Here's how you can remove the SQL table constraint for your reference and hopefully help you.1) Disable SQL for all table constraintsSelect ' ALTER TABLE ' +name+ ' nocheck
1) constraints: constraints Imposed on tables to ensure data integrity and prevent data loss in associated tables;Common constraints:-NOT NULL-UNIQUE-PRIMARY KEY-FOREIGE KEY-CHECK When creating a table, specify the constraints. You can also add or modify the table after creating the table. The constraints can be column-level or table-level;Reate table [schema.] table(Column datatype [DEFAULT expr][Column_constraint],...[Table_constraint] [,...]); 2) A
constraint UQ _stuno, delete if EXISTS (SELECT * from sysobjects WHERE name= ' uq_stuid ' and xtype= ' UQ ') Alter TABLE stuinfodrop Constraint U q_stuidgo--add unique constraint uq_stuidalter TABLE stuinfo add CONSTRAINT uq_stuid Unique (stuid)--Example 4 add default DF constrain
Tags: AST ID show Beida jade bird exec constraint for add startUse masterGo --------------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------Create a database storage location (folder)-------------------------------------------------- -----------------------------------------------------------------------------------------
Label:If there is only one table in the caseTable A:ID num1,num21 10 52 10 03 20 0Select ID, num1,num2,num1-num2 as subnum from A;In this case:The results are as follows:ID NUM1 num2 Subnum1 10 5 52 10 0 103 20 0 20In a single-table case, so that in SQL, using the minus sign "-" is no problem ( numeric fields, must not be null), and if there is a field null, then
Tags: image false strong another rule for anti-avoidance technologyFirst, SQL SERVER is a three-valued logic that predicate evaluates to True,false,unknown The standard predicates are those that follow this rule. If slary>0 returns a result that evaluates to true the row rejects false and the unknown result But not all predicate processing is like this if it appears in a check constraint that evaluates to F
CREATE TABLE Employees (EmpId int identity (empname), Empgender char (2), empage int,empemail varchar (100), empaddress varchar (+)) gocreate Table Department (depid int identity (), Depname varchar (50))--Manually delete a column (delete empaddress column) ALTER TABLE Employees drop column empaddress--Manually add a column (add a column empaddr varchar) ALTER TABLE Employees add empaddr varchar (1000)--Manually modify the data type of empemail (varchar) ALTER TABLE employess ALTER COLUMN EMPADD
Label: 1 1) disables SQL for all table constraints2 Select 'ALTER TABLE'+Name+'nocheck constraint All' fromsysobjectswhereType='U' 3 4 2) Delete all table data for SQL5 Select 'TRUNCATE TABLE'+Name fromsysobjectswhereType='U' 6 7 3) restore SQL for all table constraints8 Select 'ALTER TABLE'+Name+'Check Constraint a
Tags: drop 9.png judge Ctr ble off using LTE addModify the default constraint using the SSMS database management tool1, connect the database, select Data table-Right click-"Select Design." 2, in the Table Designer window-"Select the data column to modify-" in the column properties found in the default value binding-"to modify. 3. Click Save (or Ctrl+s)-"Close Table Designer-" Refresh table-"Reopen Table Designer to view. To modify the default
Label:Http://blog.sina.com.cn/s/blog_63a48d250100smu3.html--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
Label:Iv. SQL PRIMARY KEY constraint PRIMARY KEY constraint uniquely identifies each record in a database table The primary key must contain a unique value Primary key columns cannot contain NULL values Each table should have a primary key, and each table can have only one primary key The following
ConstraintName? 1 ALTER TABLE student DROP CONSTRAINT PK__student__2F36BC5B772B9A0B 10. Adding field constraints Format: ALTER TABLE tablename ADD constraint ConstraintName primary key (column_name) --Example 1? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 ALTER TABLE stuInfo ADD CONSTRAINT
treated as false, and in a check constraint, unknown is treated as true. This is why we can also insert a null value into the field if we set the value of a field to be greater than or equal to 0, because the logical result of null>=0 in the check constraint is unknown treated as true.It is important to note that in t
BOOLIssqlreferenceconstraintexception ( ThisException Exception) { varBaseex =exception. GetBaseException (); if(Baseex isSqlException) { if((Baseex asSqlException). Number = =547) { return true; } Else { return false; } } Else if(Baseex! =NULL!Object. ReferenceEquals (Exception, Baseex))//if the base
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