How sql modifies the table structure
This article will explain in detail how to modify the table structure of sql. The editor thinks it is very practical, so I share it with you as a reference. I hope you can get something after reading this article.
The operations to modify the table structure in SQL include adding table fields, deleting table fields, renaming fields, changing field types, setting primary keys, and adding not null constraints. Let's take a look at these operations separately.
1. Add table fields
ALTER TABLE table name ADD field name field type DEFAULT NULL
2. Delete the table field
Alter table table name drop field name
Example:
Alter table user drop name
3. Rename the field
Alter table table name rename old field name to new field name
Example:
Alter table user rename oldname to newname
4. Change the field type
ALTER TABLE table name
The type after the ALTER COLUMN field name is changed
Example:
Alter table user alter name varchar (50)
5. Set the primary key
ALTER TABLE table name
ADD CONSTRAINT primary key name PRIMARY KEY (field name)
6. Add not null constraint
ALTER TABLE table name
ALTER COLUMN Field name Field Type NOT NULL
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