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
StatementThe 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
StatementThe 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 table_nameVALUES (value1, value2);-- Insert into columns by name:INSERT INTO table_name (column1, column2)VALUES (value1, value2);
ALTER TABLE
StatementThe 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_nameADD column_name datatype;
DELETE
StatementThe 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_nameWHERE some_column = some_value;
UPDATE
StatementThe 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_nameSET column1 = value1, column2 = value2WHERE some_column = some_value;
AND
OperatorThe AND
operator allows multiple conditions to be combined. Records must match both conditions that are joined by AND
to be included in the result set. The given query will match any car that is blue and made after 2014.
SELECT modelFROM carsWHERE color = 'blue'AND year > 2014;
AS
ClauseColumns or tables can be aliased using the AS
clause. This allows columns or tables to be specifically renamed in the returned result set. The given query will return a result set with the column for name
renamed to movie_title
.
SELECT name AS 'movie_title'FROM movies;
OR
OperatorThe OR
operator allows multiple conditions to be combined. Records matching either condition joined by the OR
are included in the result set. The given query will match customers whose state is either 'CA'
or 'NY'
.
SELECT nameFROM customersWHERE state = 'CA'OR state = 'NY';
%
WildcardThe %
wildcard can be used in a LIKE
operator pattern to match zero or more unspecified character(s). The given query will match any movie that begins with The
, followed by zero or more of any characters.
SELECT nameFROM moviesWHERE name LIKE 'The%';
SELECT
StatementThe SELECT *
statement returns all columns from the provided table in the result set. The given query will fetch all columns and records (rows) from the movies
table.
SELECT *FROM movies;
_
WildcardThe _
wildcard can be used in a LIKE
operator pattern to match any single unspecified character. The given query will match any movie which begins with a single character, followed by ove
.
SELECT nameFROM moviesWHERE name LIKE '_ove';
ORDER BY
ClauseThe ORDER BY
clause can be used to sort the result set by a particular column either alphabetically or numerically. It can be ordered in two ways:
DESC
is a keyword used to sort the results in descending order.ASC
is a keyword used to sort the results in ascending order (default).SELECT *FROM contactsORDER BY birth_date DESC;
LIKE
OperatorThe LIKE
operator can be used inside of a WHERE
clause to match a specified pattern. The given query will match any movie that begins with Star
in its title.
SELECT nameFROM moviesWHERE name LIKE 'Star%';
DISTINCT
ClauseUnique values of a column can be selected using a DISTINCT
query. For a table contact_details
having five rows in which the city
column contains Chicago, Madison, Boston, Madison, and Denver, the given query would return:
Chicago
Madison
Boston
Denver
SELECT DISTINCT cityFROM contact_details;
BETWEEN
OperatorThe BETWEEN
operator can be used to filter by a range of values. The range of values can be text, numbers, or date data. The given query will match any movie made between the years 1980 and 1990, inclusive.
SELECT *FROM moviesWHERE year BETWEEN 1980 AND 1990;
LIMIT
ClauseThe LIMIT
clause is used to narrow, or limit, a result set to the specified number of rows. The given query will limit the result set to 5 rows.
SELECT *FROM moviesLIMIT 5;
NULL
ValuesColumn values can be NULL
, or have no value. These records can be matched (or not matched) using the IS NULL
and IS NOT NULL
operators in combination with the WHERE
clause. The given query will match all addresses where the address has a value or is not NULL
.
SELECT addressFROM recordsWHERE address IS NOT NULL;
WHERE
ClauseThe WHERE
clause is used to filter records (rows) that match a certain condition. The given query will select all records where the pub_year
equals 2017
.
SELECT titleFROM libraryWHERE pub_year = 2017;