8. SQL Basic collation (constraint)

Constraining primary KEY constraints

Prevent errors in new data, binding, the role of a unique flag, in addition to prevent accidental addition of duplicate content (Null values are not allowed)

1. Right button-design-set primary key

2. Set when creating a table

Code int PRIMARY KEY,

3, can set self-growth function

Code int PRIMARY Key identity (+)

4, in the self-growth environment to delete one row, other rows are unaffected

5, can speed up the query, slow down the new and modified speed

FOREIGN KEY constraints

Design-relationship-add-table and column specification-two columns to contact

The primary key needs to be set first, and then a foreign key is established on the basis of the primary key to control the other foreign keys through the primary key.

8. SQL Basic collation (constraint)