Common commands of database 2

1. Field null And not null.

• null： The expression field can be null
• not null： Indicates that the field cannot be null

2. auto number (auto_increment), And primary key (primary key) Use at the same time .

3. Primary key constraint .

Each table will have a primary key , And there is only one primary key , The primary key is the unique identifier of this table , It can be used to represent all the data in the table , The primary key cannot be repeated , And the primary key can be multiple fields , A primary key formed by multiple fields is called a federated primary key .

In general , The tables we create usually contain ID This type of field is set as primary key , Each table is on Java Corresponding to an entity class , The primary key is automatically a non empty constraint .

When we create a table , For example, the fields in the table are ID, And set it to auto_increment primary key, At this time, when we add records to the table , Don't add to the table by yourself ID The record of , So our SQL Data will be automatically added , The primary key value of each record is added (ID) Go down according to the record of the upper layer .

4. Unique constraint .

It is used to ensure the uniqueness of records in the table , It can not be a primary key , It can be for null, And each table can have multiple unique constraints .

5. Default constraint .

After adding the keyword of the default constraint to the field in the create table , If you go to the table insert Record , No non specified records are added to the fields of the default constraint , Then the system automatically adds a default record .

6. Foreign key constraints .

Create constraints on two tables , Keyword is forergn key. One is the father table , One is a sub table , They have the same search engine ,mysql The default storage engine for is Innodb. Foreign key column ( Sub table ) And reference columns must have the same index , If not, then mysql It will be indexed automatically .

Create the parent table ：

Create child tables ：

Add the record of the parent table field ：

Add records of sub table fields ：

In the picture above ,class A table is a reference column , That is, the parent table ,student The table is a foreign key column , That is, sub table . Which field id Is a keyword used as a foreign key constraint , Add to the sub table id by 1 perhaps 2, It represents the corresponding class in the student's parent table id, That is to say suyi、lisi It's a class 2-101 Of the students ,niuer、zhangsan yes 1-101 Of the students .

among class Corresponding student It's a many to one relationship (one to many),student Corresponding class It's a one-on-one relationship (one to one).

Be careful ： When we want to delete records , Also consider whether the table you want to delete is related to other tables , If it exists, the records in the parent table cannot be deleted directly , First delete the associated records in the sub table , Then you can delete the records in the parent table .

• Constraints on a data column , Called column level constraint
• Constraints on multiple data columns , Table level constraints
• Column level constraints can be declared at the time of column definition , You can also declare... After the column definition , Table level constraints can only be declared after a column definition

7. Change the name of the created table .

rename table table_name to new_table_name;

8. Modify the structure of the table .

• Add a field after the existing field ：alter table table_name add col_name not null after table_col_name;
• Add a field at the first position in the table ：alter table table_name add col_name not null first table_col_name;

9. Add multiple columns .

• alter table table_name add (col_name1,col_name2);

10. Delete the fields in the table .

• alter table table_name drop table_col_name;

11. Delete multi column fields .

• alter table table_name drop table_col_name1,drop table_col_name2;

12. Add constraints to unconstrained table fields .

• alter table table_name add Constraint keywords  (table_col_name);

13. Delete constraints for table fields with constraints .

• 1：alter table table_name drop index table_col_name;
• 2：alter table table_name drop  Constraint keywords ;

14. Change the type of field .

• alter table table_name MODIFY table_col_name Field type first;

Be careful ： If the modified field type changes from large to small , And there is data in this field in the table , Then it may cause data loss .

15. Modify the name of the field in the table .

• alter table table_name change table_col_name new_col_name Field type ;

16. Update the new record of the field in the table .

• update table_name set table_col_name=table_col_name+ New record ;

17. Paging query limit.

• select * from table_name limit m,n;
• m： Subscript from the existing table as m The position begins
• n： Output n Bar record