Integrity constraints of SQL Server database

There are four types of constraints in the database to ensure integrity ：

Domain integrity ——  Constraints on Columns

Entity integrity ——  Constraints on rows （ Cannot use duplicate lines ）

Citation integrity ——  A column value in one table is derived from another table

Custom integrity （ This may be a little esoteric , Not understood ）

1. Constraint methods to ensure domain integrity ： Check constraint 、 Default constraint 、 Non empty constraint

Check constraints, that is Check constraint , Another point is to control conditions , But the writing of conditions cannot be so casual , There are also norms , As for this specification ,,, At present, I will use ： Gender choice ：sex = 'men' or sex = 'women', Or is it  sex IN('men' , 'women'), For mail @ Requirements for symbols ：email  “%@%”, It means that there must be @ Symbol ,% Indicates that there are any bytes , Judgment of size relationship ： for example hours > 0, There is also a judgment of length ：length(log) >= 6 （ among sex、email、hours、log Are the names of columns in the table ）.

The default constraint is when no value is entered , Display the default value of the setting , The setting position is in the default value or binding of the following column properties .

A non empty constraint is whether a table is allowed to be empty when it is designed .

2. Constraint method to ensure entity integrity ： Unique constraint 、 Primary key constraint 、 identifier

Both the unique constraint and the primary key constraint are to ensure that the values in a column cannot be repeated , Primary key constraints can set multiple columns as primary keys , This increases the possibility , That is, as long as the columns of the primary key are not exactly the same .

identifier , The first must be int type , And the identifier cannot be edited , It is automatically generated . If a column is deleted , Its identification column cannot be automatically updated .

3. Constraint methods to ensure referential integrity ： Foreign key constraints

It's a watch （ From the table ） One or more columns in （ From the key ） Is quoted from another table （ Main table ） Of , That is, from this column in the table （ I.e. slave key ） The data in must be existing in the main table （ This also determines why the values in the main table should be entered first when entering data ）. The setting method is ： Right click the column name ->  Relationship -> add to -> Table and column specifications , Then set the primary key table from the key table , And the primary and secondary keys .

 

Precautions after the establishment of main and foreign keys ：

1. When there is no corresponding record in the main table , Cannot add record to slave table

2. You cannot change the value in the main table to isolate the records in the child table

3. When there are records corresponding to the main table in the sub table , The row cannot be deleted from the main table

4. Before deleting the main table , Delete the sub table first