Use SQL to change the data type of a table's columns and add new columns and constraints

Use SQL to change the data type of a table's columns and add new columns and Constraints

Modify a column of a database table or add a column

--Add a column

ALTER table name ADD column name VARCHAR (+) NULL

--Delete a column

ALTER table table name drop column name

--Modify a column

ALTER TABLE name ALTER COLUMN name VARCHAR (+) NULL

--Modify the type of a column

ALTER TABLE name ALTER COLUMN name VARCHAR (40)-

-Add a PRIMARY KEY constraint

ALTER TABLE name ADD CONSTRAINT constraint name primary key (column name)

--Add UNIQUE constraints

ALTER TABLE name ADD CONSTRAINT constraint name unique (column name)

--Add a DEFAULT constraint

ALTER TABLE name ADD CONSTRAINT constraint name default (content) for column name

--Add a CHECK constraint

ALTER TABLE name ADD constraint constraint name check (content)

--Add a FOREIGN KEY constraint

ALTER TABLE name ADD CONSTRAINT constraint name foreign key (column name) references another table name (column name)

--Delete Constraint

ALTER TABLE name DROP CONSTRAINT constraint name

--vertical connection of two tables

Select *from stuinfo unionselect *from stumarks

--Rename, table name

EXEC sp_rename ' original table name ', ' Change table name '--Rename,

Column name exec sp_rename ' table name. Original column name ', ' changed column name ', ' column '

Use SQL to change the data type of a table's columns and add new columns and constraints