"2017-03-09" SQL Server Database Foundation, four constraints

One, the difference between database and memory

Database:
Some data files that are stored on your hard disk

Memory:
Some data that the computer temporarily stores

Second, the common database

. Net-sql Server

Php-mysql

Java-oreacl

Iii. How SQL Server is used

1. Create a new database

Right click on "Database" and click "New Database". In the pop-up page, name the new database and click Confirm.

2. New Table

Locate the newly created database, right-click on "Tables" in the new database and click "New Table". Edit column name, data type

Data type Note:

int integral type

Decimal (length, number of decimal digits) floating-point

Bit-Boolean type 1-true 0-false

DateTime-Time Date type

Date-only Dates

nvarchar (length) Max String type English letters and Chinese characters are equal in length, it is recommended to use nvarchar
varchar (length) String type

Text-long text

Image-Image type, with picture stream

Iv. Four kinds of constraint relationships in database

1. PRIMARY KEY constraint:
Set a column as the primary key column

Duplicate is not allowed, NULL is not allowed, a table has only one primary key column

In the new Table edit page, right-click the column to select Set Primary key.

2. Foreign KEY constraints: (the most important constraint for establishing relationships between tables and tables)
One of the columns of the current table is constrained by one of the primary key columns/unique columns of the other table
The contents of the column in which the current table is constrained are not allowed to exceed the contents of the column that constrains it

To constrain columns of other tables as primary constraint columns, this column must be a primary key column/Unique column

On the constrained table: Right-click on the "Design", right-click on the constrained column "relationship", click "Add", on the right "table and column Specification" Set the constraint specification, click "" ... "after" Table and column Specification "

Selecting the primary key table is the constraint table, and the foreign key table is the constrained table. Confirm and save when you are finished selecting.

3, the only constraint:
Set a column to be unique, so that the data for this column cannot be duplicated

Right-click on "Index key"---"Add"----change "is unique" to "yes"

4. Check constraint:
Write an expression that constrains the range of values in a column

Right-click on "Check Constraint"--"add"---add an expression
&&-
|| -OR

"2017-03-09" SQL Server Database Foundation, four constraints