Summary of MySQL constraints

Catalog

What are constraints ？

  Constraint classification

Restrict the use of

not null constraint

unique constraint

primary key constraint

foreign key

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What are constraints ？

• In order to ensure the consistency and integrity of data ,SQL The specification imposes additional constraints on table data in a constrained manner .

• Constraints are mandatory at the table level .

• Constraints can be specified when creating tables （create table sentence ）, Or after the table is created ( adopt alter table sentence ）.

  Constraint classification

1） General classification

• not null Non empty constraint , Specifies that a field cannot be empty ;

• unique Unique constraint , Specify that a field is unique in the whole table ;

• primary key Primary key （ Non empty and unique ）

• foreign key Foreign keys

• check Check constraint （mysql I won't support it check constraint , But you can use check constraint , Without any effect ）

• default The default value is .

2） According to the restriction of the constraint column , Constraints can be divided into

• Single column constraints

• Multi column constraint

3） According to the constraint range

• Column level constraints ： It can only act on one column , Follow the definition of the column ;

• Table level constraints ： Can act on multiple columns , Not with columns , It is defined separately .

Restrict the use of

not null constraint

• The non empty constraint is used to ensure that the value of the current column is not empty , Non empty constraints can only appear on columns of table objects ;

• null Characteristics of the type ： All types of values can be null, Include int,float And so on

• An empty string “” and 0 None of them is equal to null.

• establish not null constraint ：

• increase not null constraint

• 

• Cancel null constraint

   

unique constraint

Unique constraint , Multiple null values are allowed .

• The same table can have multiple unique constraints , Constraints of multiple column combinations .

• When creating unique constraints , If you don't give a unique constraint name , The default is the same as the column name ;

• mysql A unique index will be created by default for columns with unique constraints

• Create a unique constraint

• 

• Add unique constraints

• 

• Delete constraints

 

primary key constraint

• A primary key constraint is equivalent to a unique constraint + A combination of nonnull constraints , Primary key constraint cannot be duplicated , Null values are not allowed .

• If it is the primary key constraint of multi column combination , Then none of these columns are allowed to be null , And the combined value is not allowed to repeat ;

• Only one primary key is allowed per table , You can create a primary key constraint at the column level , You can also create... At the table level ;

• mysql The primary key name of is always primary, When creating a primary key constraint , By default, the system will establish a unique index on the column and column combination .

• Create constraints

• 

• Delete primary key constraint

alter　table dep drop primary key;

• Add primary key constraint

alter table dep add primary key(name, pwd);

• Modify the primary key constraint

alter table dep modify id int primary key;

foreign key

• A foreign key constraint guarantees referential integrity between one or two tables , A foreign key is a reference relationship between two fields of a table or two fields of two tables .

• The foreign key value of the slave table must be found in the master table or empty . When the records of the master table are referenced by the slave table , Records in the main table will not be allowed to be deleted , If you want to delete data , You need to delete the data that depends on this record from the table first , Then you can delete the data of the main table .

• Reference column of foreign key constraint , Only primary key or unique key constrained columns can be referenced in the main table .

• The same table can have multiple foreign key constraints .

• Create foreign key constraints

• 

• Create a multi column foreign key combination , Table level constraints must be used

• 

• Delete foreign key constraint

   

• Add foreign key constraints

   

• foreign key Keywords for constraints