Modify Table field name/Field Length/field type in mysql

In mysql, we only need to use alter to modify the data table fields. Below I will introduce how to modify the table field name, field length, and field type in mysql, for more information, see.

Let's take a look at common methods.

The simple Syntax of MySql is commonly used, but it is not easy to remember. Of course, these SQL syntaxes are basically common in various databases. Listed below:

1. Add a field

Alter table user add COLUMN new1 VARCHAR (20) default null; // add a field, empty by DEFAULT
Alter table user add COLUMN new2 VARCHAR (20) not null; // add a field, which cannot be blank by default.

2. delete a field

Alter table user drop column new2; // delete a field

3. modify a field

Alter table user MODIFY new1 VARCHAR (10); // MODIFY the type of a field
Alter table user CHANGE new1 new4 int; // modify the name of a field.

// Primary key

Alter table tabelname add new_field_id int (5) unsigned default 0 not null auto_increment, add primary key (new_field_id );

// Add a new column

Alter table t2 add d timestamp;
Alter table infos add ex tinyint not null default '0 ′;

// Delete a column

Alter table t2 drop column c;

// Rename a column

Alter table t1 change a B integer;

// Change the column type

Alter table t1 change B bigint not null;
Alter table infos change list tinyint not null default '0 ′;

// Rename the table

Alter table t1 rename t2;

Add Index

Mysql> alter table tablename change depno int (5) not null;
Mysql> alter table tablename add index name (field name 1 [, field name 2…]);
Mysql> alter table tablename add index emp_name (name );

Index with primary keywords

Mysql> alter table tablename add primary key (id );

Add an index with unique conditions

Mysql> alter table tablename add unique emp_name2 (cardnumber );

Delete An index

Mysql> alter table tablename drop index emp_name;

Add field:

Mysql> alter table table_name ADD field_name field_type;

Modify the original field name and type:

Mysql> alter table table_name CHANGE old_field_name new_field_name field_type;

Delete field:

Mysql> alter table table_name DROP field_name;

Modify the Field Length in mysql

Alter table name modify column field name type;

For example

The name field of the user table in the database is varchar (30)

Available

Alter table user modify column name varchar (50 );