Statements to modify table structure in sql

The statements in SQL that modify the table structure include: ALTER TABLE table_name ADD column_name data_type Add column ALTER TABLE table_name ALTER COLUMN column_name data_type Modify column data type ALTER TABLE table_name DROP COLUMN column_name Delete column ALTER TABLE table_name RENAME COLUMN old_column_name TO new_column_

Statements to modify table structure in SQL

Modifying table structure is a common operation in SQL, which allows you to Change a table's columns, data types, and constraints. The following are some commonly used statements:

1. Add columns

ALTER TABLE table_name ADD column_name data_type

For example:

<code>ALTER TABLE employees ADD salary INT</code>

2. Modify columns

ALTER TABLE table_name ALTER COLUMN column_name data_type

For example:

<code>ALTER TABLE employees ALTER COLUMN salary DECIMAL(10,2)</code>

3. Delete column

ALTER TABLE table_name DROP COLUMN column_name

For example:

<code>ALTER TABLE employees DROP COLUMN bonus</code>

4. Change column Name

ALTER TABLE table_name RENAME COLUMN old_column_name TO new_column_name

For example:

<code>ALTER TABLE employees RENAME COLUMN first_name TO name</code>

5. Add constraints

Primary key:

##ALTER TABLE table_name ADD PRIMARY KEY (column_name)

For example:

<code>ALTER TABLE employees ADD PRIMARY KEY (employee_id)</code>

Foreign key:

ALTER TABLE table_name ADD FOREIGN KEY (column_name) REFERENCES referenced_table(column_name)

For example:

<code>ALTER TABLE orders ADD FOREIGN KEY (customer_id) REFERENCES customers(customer_id)</code>

Unique constraints:

ALTER TABLE table_name ADD UNIQUE (column_name)

For example:

<code>ALTER TABLE employees ADD UNIQUE (email)</code>

6. Delete constraints

Primary key:

ALTER TABLE table_name DROP PRIMARY KEY

Foreign key:

ALTER TABLE table_name DROP FOREIGN KEY column_name

Unique constraint:

ALTER TABLE table_name DROP INDEX index_name (where index_name is the name of the unique constraint)

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