IST 210 More SQL Todd Bacastow IST 210

IST 210 More SQL Todd Bacastow IST 210: Organization of Data

IST 210 Simple Join Consider: Likes (drinker, beer) Sells (bar, beer, price) SELECT drinker, bar FROM Likes, Sells WHERE Likes. beer=Sells. beer; n What’s happening here? ?

IST 210 Examples Employees: Name Employee_ID 01 Hansen, John 02 Shara, Tove 03 Seven, Stephen 04 Pettersen, Kari Orders: Product Employee_ ID 234 Printer 01 657 Table 03 865 Chair 03 Prod_ID

IST 210 Who has ordered a product, and what did they order? SELECT Employees. Name, Orders. Product FROM Employees, Orders WHERE Employees. Employee_ID=Orders. Employee_ID Result Name Product Hansen, John Printer Seven, Stephen Table Seven, Stephen Chair

IST 210 Who ordered a printer? SELECT Employees. Name FROM Employees, Orders WHERE Employees. Employee_ID=Orders. Employee_ID AND Orders. Product='Printer' Result Name Hansen, John

IST 210 Other ‘Joins’ in SQL 2 n n n R R R NATURAL JOIN S CROSS JOIN S ON condition OUTER JOIN S INNER JOIN S

Alternative JOIN Constructs IST 210 n SQL 2 provides alternative ways to specify joins: SELECT * FROM Likes. beer JOIN Sells. beer ON Likes. beer= Sells. beer SELECT * FROM Likes JOIN Sells USING beer SELECT * FROM Likes NATURAL JOIN Sells n The first produces a table with two identical beer columns, remaining two produce table with single beer column.

IST 210 Outer Join n n Outerjoin = natural join with dangling tuples padded with NULLs and included in the result A tuple is dangling if it doesn’t join with any other tuple A B 1 2 3 4 OUTERJOIN B C 2 5 2 6 7 8 = A B C 1 2 5 1 2 6 3 4 NULL 7 8

IST 210 Modifiers of OUTER JOIN n n n Optional NATURAL in front Optional ON condition at end Optional LEFT, RIGHT, FULL before OUTER n n LEFT = Pad dangling tuples of R ONLY RIGHT = Pad dangling tuples of S ONLY

IST 210 Inner Join The INNER JOIN returns all rows from both tables where there is a match. If there are rows in Employees that do not have matches in Orders, those rows will not be listed. n EXAMPLE: Who has ordered a product, and what did they order? n SELECT Employees. Name, Orders. Product FROM Employees INNER JOIN Orders ON Employees. Employee_ID=Orders. Employee_ID Result Name Product Hansen, John Printer Seven, Stephen Table Seven, Stephen Chair

IST 210 Calculated Fields n Query Produce a list of monthly salaries for all staff, showing the staff number, the first and last names, and the salary details. SELECT sno, fname, lname, salary/12 Monthly_salary FROM staff; To name column, use AS clause: SELECT sno, fname, lname, salary/12 AS monthly_salary FROM staff;

IST 210 Range Search Condition - BETWEEN Query: List all staff with a salary between 20, 000 and 30, 000. SELECT sno, fname, lname, position, salary FROM staff WHERE salary BETWEEN 20000 AND 30000;

IST 210 n Range Search Condition (Cont…) BETWEEN test includes the endpoints of range. Equivalent to SELECT sno, fname, lname, position, salary FROM staff WHERE salary>=20000 AND salary <= 30000; n n BETWEEN does not add much to SQL's expressive power Also a negated version NOT BETWEEN.

IST 210 Set Membership - IN / NOT IN n Query: List all Managers and Deputy Managers. SELECT sno, fname, lname, position FROM staff WHERE position IN ('Manager', 'Deputy');

Pattern Matching - LIKE IST 210 n SQL has two special pattern matching symbols: n n n %: sequence of zero or more characters; _ (underscore): any single character. LIKE '%Glasgow%' means a sequence of characters of any length containing 'Glasgow'. Query: Find all staff with the string 'Glasgow' in their address. SELECT sno, fname, lname, address, salary FROM staff WHERE address LIKE '%Glasgow%';

IST 210 n n NULL Search Condition NULL value can not be checked by comparison operators Have to test for null explicitly using special keyword IS NULL IS NOT NULL can test for non-null values. Example query: List details of all viewings on property PG 4 where a comment has not been supplied. SELECT rno, date FROM viewing WHERE pno = 'PG 4' AND comment IS NULL;

IST 210 ORDER BY n Example Query: List salaries for all staff, arranged in descending order of salary. SELECT sno, fname, lname, salary FROM staff ORDER BY salary DESC; n n When no (ASC/DESC) is indicated, default order is ascending (ASC). Can sort on multiple columns. SELECT pno, type, rooms, rent FROM property_for_rent ORDER BY type, rent DESC;

IST 210 GROUP BY and HAVING n n Use GROUP BY clause to get sub-totals. SELECT and GROUP BY closely integrated: each item in SELECT list must be single-valued per group, and SELECT clause may only contain: n n n Column names. Aggregate functions. Constants. An expression involving combinations of the above. If WHERE is used with GROUP BY, WHERE is applied first, then groups are formed from remaining rows satisfying predicate. Considers two nulls to be equal for purposes of GROUP BY.

IST 210 Use of GROUP BY Example: Find number of staff in each branch and their total salaries. SELECT bno, COUNT(sno) AS count, SUM(salary) AS sum FROM staff GROUP BY bno ORDER BY bno; 59

IST 210 Restricted Grouping - HAVING n n HAVING clause is designed for use with GROUP BY clause to restrict groups that appear in final result table. HAVING filters groups. Column names in HAVING clause must also appear in the GROUP BY list or be contained within an aggregate function. Example: For each branch with more than 1 member of staff, find number of staff in each branch and sum of their salaries. SELECT bno, COUNT(sno) AS count, SUM(salary) AS sum FROM staff GROUP BY bno HAVING COUNT(sno) > 1 ORDER BY bno;

IST 210 Subquery with Aggregate n Example: List all staff whose salary is greater than the average salary. SELECT sno, fname, lname, position, salary FROM staff WHERE salary > (SELECT avg(salary) FROM staff); n n n Cannot write 'WHERE salary > avg(salary)' Instead, use subquery to find average salary (17000), and then use outer SELECT to find those staff with salary greater than 17000 Subquery SELECT list must consist of a single column name or expression, except for subqueries that use EXISTS.

IST 210 ANY/SOME and ALL n n n If subquery preceded by ALL, condition will only be true if it is satisfied by all values produced by subquery. If subquery preceded by ANY, condition will be true if it is satisfied by any values produced by subquery. If subquery is empty, ALL returns true, ANY returns false.

IST 210 Example of ANY and ALL n Example 1: Find staff whose salary is larger than salary of at least 1 member of staff at branch B 3. SELECT sno, fname, lname, position, salary FROM staff WHERE salary >ANY (SELECT salary FROM staff WHERE bno = 'B 3'); n Example 2: Find staff whose salary is larger than salary of every member of staff at branch B 3. SELECT sno, fname, lname, position, salary FROM staff WHERE salary > ALL (SELECT salary FROM staff WHERE bno = 'B 3');

IST 210 n Sorting a join Consider: For each branch, list names of staff who manage properties. SELECT s. bno, s. sno, fname, lname, pno FROM staff s, property_for_rent p WHERE s. sno = p. sno ORDER BY s. bno, s. sno, pno;