Joins, NULL, and Aggregation

Transcription

1 Joins, NULL, and Aggregation FCDB Dr. Chris Mayfield Department of Computer Science James Madison University Jan 29, 2018

2 Announcements 1. Your proposal is due Friday in class Each group brings three hard copies Take a look at the JMU Writing Rubric 2. HW3 will mainly involve intermediate SQL Gradiance will have several warm-up queries Review the previous three labs for the basics 3. Your db account is separate from your JMU account! Do NOT use the same password for both accounts Change your db password in psql with \password Jan 29, 2018 Joins, NULL, and Aggregation 2 of 17

3 SQL Exercises: imdb Look for Frozen in the movie table Only display movies after the year 2010 List all movies beginning with Fro List unique titles starting with Fro

4 More than one relation SELECT name FROM Movies, MovieExec WHERE title = 'Star Wars' AND producerc# = cert#; Is this a cross product or a theta join? SELECT s.name, e.name FROM MovieStar AS s, MovieExec AS e WHERE s.address = e.address; How did we disambiguate attributes? Jan 29, 2018 Joins, NULL, and Aggregation 4 of 17

5 SQL join expressions SELECT * FROM Movies, StarsIn; -- implicit product SELECT * FROM Movies CROSS JOIN StarsIn; -- explicit product SELECT * FROM MovieStar NATURAL JOIN MovieExec; -- self explanatory SELECT * FROM Movies -- theta join (a.k.a. inner join) JOIN StarsIn ON title = movietitle AND year = movieyear; Jan 29, 2018 Joins, NULL, and Aggregation 5 of 17

6 Self join revisited SELECT s1.name, s2.name FROM MovieStar AS s1 JOIN MovieStar AS s2 ON s1.address = s2.address WHERE s1.name < s2.name; What does this query do again? Advantages of join syntax Makes programmer s intent more clear Separates join conditions from others Provides syntax for outer/natural join Jan 29, 2018 Joins, NULL, and Aggregation 6 of 17

7 SQL Exercises: imdb For each Frozen what kind of movie was it? What is the plot of the movie Frozen (2013)? Hint: plot is info id = 98 in movie info

8 Outer joins in SQL -- return all movie execs, with optional stars SELECT * FROM MovieStar NATURAL RIGHT JOIN MovieExec; -- return all movies, with optional stars SELECT * FROM Movies LEFT JOIN StarsIn ON title = movietitle AND year = movieyear; -- return all stars and execs (side by side) SELECT * FROM MovieStar AS s FULL JOIN MovieExec AS e ON s.name = e.name; Jan 29, 2018 Joins, NULL, and Aggregation 8 of 17

9 See full article on CodeProject

10 NULL and UNKNOWN In SQL, attribute values may be NULL integer may be to or NULL text may be Hello or or NULL Rules for NULL 1. Any operation on NULL is NULL 2. Any comparison with NULL is UNKNOWN What is the value of: 5 + NULL NULL = NULL title IS NULL Jan 29, 2018 Joins, NULL, and Aggregation 10 of 17

11 SQL Exercises: imdb What is the budget of each movie? (if known) What is the runtime of each movie? (if known) Hint: in movie info, budget is 105, runtime is 1

12 Grouping and aggregation Aggregation functions count, sum, avg, min, max SELECT max(length) FROM Movies WHERE studioname = 'Disney'; More commonly used in groups: γ studioname,avg(length) avglen (Movies) SELECT studioname, avg(length) AS avglen FROM Movies GROUP BY studioname; Jan 29, 2018 Joins, NULL, and Aggregation 12 of 17

13 HAVING vs WHERE For each star who has appeared in at least three movies, find the earliest year in which they appeared. π starname,minyear σ cttitle 3 γ starname, min(year) minyear, count(title) cttitle StarsIn SELECT starname, min(year) AS minyear FROM StarsIn GROUP BY starname HAVING count(title) >= 3; -- selection in algebra! Jan 29, 2018 Joins, NULL, and Aggregation 13 of 17

14 SQL Exercises: imdb How many of each kind of movie? How many of each type of info?

15 Aggregation with NULLs Three ways of counting: SELECT count(*) FROM StarsIn; -- count rows SELECT count(starname) FROM StarsIn; -- count values SELECT count(distinct starname) FROM StarsIn; NULL values are ignored! SELECT name, avg(length) FROM MovieExec, Movies WHERE producerc# = cert# GROUP BY name; Does not count movies with NULL length However, count(*) of an empty table is 0 Jan 29, 2018 Joins, NULL, and Aggregation 15 of 17

16 NULLs can be confusing! SELECT a, count(b) FROM R GROUP BY a; R a b NULL NULL Returns (NULL, 0) SELECT a, sum(b) FROM R GROUP BY a; Returns (NULL, NULL) Jan 29, 2018 Joins, NULL, and Aggregation 16 of 17

17 What did the book say? 1. How are bit strings represented in SQL? (see 6.1.2) 2. What are four common uses of NULL? (see 6.1.6) 3. What if you forget a join predicate? (see 6.2.4) 4. How do subqueries with tuple expressions work? (see 6.3.3) 5. Why is duplicate elimination so expensive? (see 6.4.2) 6. What order must you list clauses in SQL? (see 6.4.6) Jan 29, 2018 Joins, NULL, and Aggregation 17 of 17