--The SQL Query Language DML--1 LIKE  LIKE allows to select character strings which have some element in common by using wild cards:  Wild cards:  “%” means “zero, one or more characters”  “_” means “one single character”

--The SQL Query Language DML--2 SELECT: Queries Over Several Tables  Which films name ends in “100”? SELECT Title FROM Film WHERE Title LIKE ‘%100’  Which films name contains the character a, b and c in the first, third and fifth positions? SELECT Title FROM Film WHERE Title LIKE ‘a_b_c%’

--The SQL Query Language DML--3 Correlated Subqueries  Subquery must be re-evaluated for each tuple in outer SELECT  Table variable is used  Find the customers who live at more than one address (assume the key of Customer is (CustomerID, Address)). SELECT Name FROM Customer C WHERE CustomerID IN (SELECT D.CustomerID FROM Customer D WHERE C.Address <> D.Address)

--The SQL Query Language DML--4 Multiple Attribute Producing Subquery  The subquery produces a table with several columns  EXISTS  true if subquery produced table has a tuple  NOT EXISTS  true if subquery produced table is empty

--The SQL Query Language DML--5 EXISTS  List the customers who live in Dublin and have a film reserved. SELECT Name FROM Customer WHERE City = ’Dublin’ AND EXISTS ( SELECT * FROM Reserved WHERE Reserved.CustomerID = Customer.CustomerID)

--The SQL Query Language DML--6 EXISTS, cont.  Often, EXISTS can be replaced with another correlation name.  List the customers who live in Dublin and have a film reserved. SELECT Name FROM Customer, Reserved WHERE City = ’Dublin’ AND Customer.CustomerID = Reserved.CustomerID

--The SQL Query Language DML--7 NOT EXISTS  List the customers who live in Dublin but have no films reserved. SELECT Name FROM Customer WHERE City = ’Dublin’ AND NOT EXISTS(SELECT * FROM Reserved WHERE Reserved.CustomerID = Customer.CustomerID)

--The SQL Query Language DML--8 Outline - The SELECT statement  Single table  Projection  Selection  Multiple tables  Cartesian product and join  Set operations  Subqueries  Optional clauses  Ordering results  Computing aggregates on groups  Additional joins

--The SQL Query Language DML--9 ORDER BY  Can sort the result of a select, using ORDER BY.  Who has reserved a film? SELECT Name FROM Customer, Reserved WHERE Customer.CustomerID = Reserved.CustomerID ORDER BY Name  Can also sort in descending order, via DESC (ASC is the default). SELECT Name FROM Customer, Reserved WHERE Customer.CustomerID = Reserved.CustomerID ORDER BY Name DESC  Only columns in the select list can be used for the ordering.

--The SQL Query Language DML--10 Select in the From Clause  The table in a from clause can itself be a select statement.  List the customers who have reserved an expensive film. SELECT CustomerID FROM Reserved,(SELECT * FROM Film WHERE RentalPrice > 4) AS Expensive WHERE Reserved.FilmID = Expensive.FilmID  A correlation name is required in such cases.

--The SQL Query Language DML--11 Aggregates  Aggregates operate on the set of values of a column of a table, and return a single value.  SUM: sum of values  AVG: average value  MAX: maximum value  MIN: minimum value  COUNT: number of values  How many reservations are there? SELECT COUNT (*) FROMReserved

--The SQL Query Language DML--12 Aggregates, cont.  What is the total rental price of all films? SELECT SUM(RentalPrice) FROMFilm  Eliminate duplicates before computing aggregate with DISTINCT  In how many cities do customers reside? SELECT COUNT(DISTINCT City) FROMCustomer

--The SQL Query Language DML--13 Aggregates, cont.  What is the average rental price of reserved films? SELECT AVG(RentalPrice) FROMReserved, Film WHEREReserved.FilmID = Film.FilmID  Find the film(s) with the highest rental price. SELECT Title FROM Film WHERE RentalPrice IN (SELECT MAX(RentalPrice) FROM Film)

--The SQL Query Language DML--14 GROUP BY  The table can be partitioned into several groups specifying GROUP BY  Each group has the same attribute values for the indicated columns. Name Age Pam4 Pat1 Joe3 2 Pat1Pam4 Joe3 2 Groups GROUP BY Name

--The SQL Query Language DML--15 GROUP BY, cont.  Aggregate is applied to each group.  What is the oldest age for each name? SELECT Name, MAX(Age) FROMPerson GROUP BY Name Name Age Pam4 Pat1 Joe3 2 Groups Pam4 Pat1 Joe3 2 MAX is 1 MAX is 4 MAX is 3 Pam4 Pat1 Joe3 Name Max(Age)

--The SQL Query Language DML--16 GROUP BY, cont.  What is the average rental price of reserved films, by kind? SELECT Kind, AVG(RentalPrice) FROMReserved, Film WHERE Reserved.FilmID = Film.FilmID GROUP BY Kind

--The SQL Query Language DML--17 HAVING  Individual groups can be eliminated by using HAVING.  List the names and average age per name, as long as the average is more than 2. Name Age Pam4 Pat1 Joe3 2 Groups Pam4 Pat1 Joe3 2 AVG is 1 AVG is 4 AVG is 2.5 Pam4.0 Joe2.5 Name AVG(Age) SELECT Name, AVG(Age) FROMPerson GROUP BY Name HAVING AVG(Age) > 2

--The SQL Query Language DML--18 HAVING, cont.  Columns in HAVING clause must appear in the GROUP BY (or be contained within an aggregate).  List the customers whose average rental price for reserved films is greater than $3. SELECT Name FROMCustomer, Reserved, Film WHERE Customer.CustomerID = Reserved.CustomerID AND Reserved.FilmID = Film.FilmID GROUP BY Name HAVING AVG(RentalPrice) > 3  Note, RentalPrice appears within an aggregate

--The SQL Query Language DML--19 Summary  A query in SQL can consist of up to six clauses, but only the first two, SELECT and FROM, are mandatory.  The clauses are specified in the following order.  SELECT column(s)  FROM table list  WHERE condition  GROUP BY grouping column(s)  HAVING group condition  ORDER BY sort list

--The SQL Query Language DML--20 Summary, cont.  The query is evaluated in a different order.  The tables in the from clause are combined using Cartesian products.  The where predicate is then applied.  The resulting tuples are grouped according to the group by clause.  The having predicate is applied to each group, possibly eliminating some groups.  The aggregate(s) are applied to each remaining group.  The select clause is performed last (followed by order clause).

--The SQL Query Language DML--21 Order of Clause Evaluation 1.FROM 2.WHERE – optional 3.GROUP BY - optional, defaults to 1 group 4.HAVING – optional 5.SELECT - (generalized) projection 6.ORDER BY - optional