Column Constraints
Column constraints are the rules applied to the values of individual columns:
PRIMARY KEYconstraint can be used to uniquely identify the row.UNIQUEcolumns have a different value for every row.NOT NULLcolumns must have a value.DEFAULTassigns 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 creates 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:
- Insert into columns in order.
- Insert into columns by name.
-- Insert into columns in order:
INSERT INTO table_name
VALUES (value1, value2);
-- Insert into columns by name:
INSERT INTO table_name (column1, column2)
VALUES (value1, value2);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.
ALTER TABLE table_name
ADD column_name datatype;DELETE Statement
The DELETE statement is used to delete records (rows) in a table. 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 includes a SET clause that indicates the column to edit and a WHERE clause for specifying the record(s).
UPDATE table_name
SET column1 = value1, column2 = value2
WHERE some_column = some_value;