Detailed explanation of alter field of MySQL Foundation

1、 First create a table testalter_tbl

    create table testalter_tbl
    (
     i INT,
     c CHAR(1)
    );

2、 Delete , Add or modify table fields

Delete field i

alter table testalter_tbl drop i

Be careful : If there is only one field left in the data table, it cannot be used DROP To delete fields .

Add fields i

Use ADD Clause to add columns... To the data table , The following examples are shown in table testalter_tbl Add i Field , And define data types :

ALTER TABLE testalter_tbl ADD i INT;

Be careful : After executing the above order ,i Fields are automatically added to the end of the data table fields .

You can also specify where to add fields , By using MySQL Keywords provided FIRST ( Set the first column ), AFTER Field name （ Set after a field ）.

ALTER TABLE testalter_tbl DROP i;# Delete i This column 
ALTER TABLE testalter_tbl ADD i INT FIRST;# hold i This column is added to the first column 

ALTER TABLE testalter_tbl DROP i;# Delete i This column 
ALTER TABLE testalter_tbl ADD i INT AFTER c;# hold i This column is added to c after 

Be careful :FIRST and AFTER Keywords can be used for ADD And MODIFY Clause , So if you want to reset the location of data table fields, you need to use DROP Delete the field and use ADD To add fields and set the location .

3、 Modify field type and name

If you need to change the field type and name , You can ALTER Use in command MODIFY or CHANGE Clause .

Put field c Type from CHAR(1) Change it to CHAR(10)

ALTER TABLE testalter_tbl MODIFY c CHAR(10);

Use CHANGE Clause modification

ALTER TABLE testalter_tbl CHANGE i j BIGINT;# Can pass change Change the field name and field type 

4、 ALTER TABLE Yes Null The effect of values and defaults

When you modify fields , You can specify whether to include values or whether to set default values .

ALTER TABLE testalter_tbl 
     MODIFY j BIGINT NOT NULL DEFAULT 100;

Be careful : If you don't set the default value ,MySQL It will automatically set the default value of this field to NULL.

Modify the default

ALTER TABLE testalter_tbl ALTER i SET DEFAULT 1000;

Delete the default value of the field

ALTER TABLE testalter_tbl ALTER i DROP DEFAULT;

Modify the name of the table

ALTER TABLE testalter_tbl RENAME TO alter_tbl;