HAVING clause: This clause is used in association with GROUP BY clause. It is applied
to the each group of result or the entire result as single group and much similar as
WHERE clause, the only difference is you cannot use it without GROUP BY clause
Syntax: SELECT column_name(s)
FROM table_name
GROUP BY column_name
HAVING condition;
ORDER BY clause: This clause is to dene the order of the query output either in
ascending (ASC) or in descending (DESC) order. Ascending (ASC) is the default one
but descending (DESC) is set explicitly.
Syntax: SELECT column_name(s)
FROM table_name
WHERE condition
ORDER BY column_name ASC|DESC;
USING clause: USING clause comes in use while working with SQL Joins. It is used
to check equality based on columns when tables are joined. It can be used instead
ON clause in Joins.
Syntax: SELECT column_name(s)
FROM table_name
JOIN table_name
USING (column_name);
Question #6) Why do we use SQL constraints? Which constraints we can use while
creating database in SQL?
Constraints are used to set the rules for all records in the table. If any constraints get
violated then it can abort the action that caused it.
Constraints are dened while creating the database itself with CREATE TABLE statement
or even after the table is created once with ALTER TABLE statement.
There are 5 major constraints are used in SQL, such as
NOT NULL: That indicates that the column must have some value and cannot be left
null
UNIQUE: This constraint is used to ensure that each row and column has unique
value and no value is being repeated in any other row or column
PRIMARY KEY: This constraint is used in association with NOT NULL and UNIQUE
constraints such as on one or the combination of more than one columns to identify
the particular record with a unique identity.
FOREIGN KEY: It is used to ensure the referential integrity of data in the table and
also matches the value in one table with another using Primary Key
Menu
