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Column Constraints

Column constraints are the rules applied to the values of individual columns:

  • PRIMARY KEY constraint can be used to uniquely identify the row.
  • UNIQUE columns have a different value for every row.
  • NOT NULL columns must have a value.
  • DEFAULT assigns a default value for the column when no value is specified.

There can be only one PRIMARY KEY column per table and multiple UNIQUE columns.

CREATE TABLE student ( id INTEGER PRIMARY KEY, name TEXT UNIQUE, grade INTEGER NOT NULL, age INTEGER DEFAULT 10 );

CREATE TABLE Statement

The CREATE TABLE statement is used to create a new table in a database. It allows one to specify the name of the table and the name of each column in the table.

CREATE TABLE table_name ( column1 datatype, column2 datatype, column3 datatype );

INSERT Statement

The INSERT INTO statement is used to add a new record (row) to a table.

It has two forms as shown in the code block:

  • Insert values based on the order of the columns in the table.
  • Define the columns to insert values into.
-- Insert into columns in order: INSERT INTO table_name VALUES (value1, value2, value3); -- Insert into columns by name: INSERT INTO table_name (column1, column2, column3) VALUES (value1, value2, value3);

ALTER TABLE Statement

The ALTER TABLE statement is used to modify the columns of an existing table. When combined with the ADD COLUMN clause, it is used to add a new column to a table.

-- Syntax: ALTER TABLE table_name ADD column_name datatype; -- Example: ALTER TABLE employees ADD first_name TEXT;

DELETE Statement

The DELETE statement is used to delete records (rows) in a table. This statement does not delete the whole table.

Inside, the WHERE clause specifies which record or records that should be deleted. If the WHERE clause is omitted, all records will be deleted.

DELETE FROM table_name WHERE some_column = some_value;

UPDATE Statement

The UPDATE statement is used to edit records (rows) in a table. It usually includes a SET clause that indicates the column to edit and a WHERE clause for specifying which record(s) should be updated.

UPDATE table_name SET column1 = value1, column2 = value2 WHERE some_column = some_value;