2021.7.22 note constraints

summary

Constraint classification

not null

Set when creating table , After setting, if a single element is inserted, an error will be reported

unique

Limit the uniqueness of the value , however null No repetition

Two level constraint unique

not null And unique Joint constraint

Primary key

primary key, Cannot repeat and as null

There can only be one primary key

Can unite primary key

ps:auto_increment Is self increasing , It can be used with primary keys , Realize automatic generation of self incrementing primary key value

Foreign keys

Storage engine