Lecture 8. Monday, September 29, 2014

Transcription

1 Lecture 8 Monday, September 29, 2014

2 UNION UNION allows us to combine two or more SELECT statements Example o If we want to find CS and MTH majors o One way: SELECT * FROM students WHERE major IN('CS','MTH');! o Another (inefficient) way: SELECT * FROM students WHERE major = 'CS' UNION SELECT * FROM students WHERE major = 'MTH';! 1

3 UNION NOTE: Make sure that if your SELECT statements are from different sources that the columns are selected in the same order Example: o Suppose we have two tables, with the fields defined in the order listed A: id, lastname, firstname B: id, firstname, lastname o We run the query: SELECT * FROM a UNION SELECT * FROM b;! o The result would have the field names from table A, but the firstname field from B would appear in the column lastname of the results, and the lastname field from B would appear in the column firstname of the results 2

4 UNION Just like you learned in Set Theory & Logic, UNION does not duplicate items in a set (or, in this case, records) Thus, SELECT * FROM a UNION SELECT * FROM a is the equivalent of SELECT * FROM a! 3

5 Naming Convention When we do JOINs, it is possible we will have ambiguous field names Example: o Suppose we have two tables A: id, name B: id, name o If we join these by their common id, we have two fields with name o We will need to refer to them in the tablename.fieldname notation o To reference the name from the table B, we would reference it with B.name 4

6 JOINs JOIN allow us to do combine two or more related tables Unlike subquerying with IN(), using JOIN allows us to have the fields returned by a SELECT statement There are seven different ways that we can join information from two tables, depending upon which records we want out of the JOIN! 5

7 Example Tables A id firstname 1 Fred 1 Peggy 2 Louise 3 Pei 4 John 5 Anthony 7 Ron 9 Christian B id lastname 1 Sullivan 4 Koch 4 Harrison 5 Kapolka 6 Gapinski 7 Pryor 8 Bracken 6

8 A (intersect) B A (intersect) B would only include records that exist for the related fields in both A and B 7

9 A (intersect) B SELECT * FROM a JOIN b USING (id);! id firstname lastname 1 Fred Sullivan 1 Peggy Sullivan 4 John Koch 4 John Harrison 5 Anthony Kapolka 7 Ron Pryor 8

10 A A would include all records in A o If the corresponding record exists in B, information from B will be included o If the corresponding record does not exist in B, the data for fields associated with B would be NULL 9

11 A SELECT * FROM a LEFT JOIN b USING (id);! id firstname lastname 1 Fred Sullivan 1 Peggy Sullivan 2 Louise NULL 3 Pei NULL 4 John Koch 4 John Harrison 5 Anthony Kapolka 7 Ron Pryor 9 Christian NULL 10

12 A would include all records in A that do not appear in B A B 11

13 B A SELECT * FROM a LEFT JOIN b USING (id) WHERE b.id IS NULL;! id firstname lastname 2 Louise NULL 3 Pei NULL 9 Christian NULL 12

14 B A would include all records in B o If the corresponding record exists in A, information from A will be included o If the corresponding record does not exist in A, the data for fields associated with A would be NULL 13

15 B SELECT * FROM a RIGHT JOIN b USING (id);! id firstname lastname 1 Fred Sullivan 1 Peggy Sullivan 4 John Koch 4 John Harrison 5 Anthony Kapolka 6 NULL Gapinski 7 Ron Pryor 8 NULL Bracken 14

16 A would include all records in B that do not appear in A B A 15

17 B A SELECT * FROM a RIGHT JOIN b USING (id) WHERE a.id IS NULL;! id firstname lastname 6 NULL Gapinski 8 NULL Bracken 16

18 Would include all records from A and B A (union) B If any records in A are not also in B, the fields associated with B would be NULL If any records in B are not also in A, the fields associated with A would be NULL 17

19 A (union) B Unlike most SQL DMBSs, you cannot do an FULL OUTER JOIN, so you must do a UNION with two SELECT statements Again, you must make sure the fields come out in the same order SELECT id, firstname, lastname FROM a LEFT JOIN b USING (id) UNION SELECT id, firstname, lastname FROM a RIGHT JOIN b USING (id);! id firstname lastname 1 Fred Sullivan 1 Peggy Sullivan 2 Louise NULL 3 Pei NULL 4 John Koch 4 John Harrison 5 Anthony Kapolka 6 NULL Gapinski 7 Ron Pryor 8 NULL Bracken 9 Christian NULL 18

20 A(intrsct)B A(union)B Would include any record that is not in both A and B For records in A that are not also in B, the fields associated with B would be NULL For records in B that are not also in A, the fields associated with A would be NULL 19

21 A(intrsct)B A(union)B SELECT id, firstname, lastname FROM a LEFT JOIN b USING (id) WHERE b.id IS NULL UNION SELECT id, firstname, lastname FROM a RIGHT JOIN b USING (id) WHERE a.id IS NULL;! id firstname lastname 2 Louise NULL 3 Pei NULL 6 NULL Gapinski 8 NULL Bracken 9 Christian NULL 20

22 Fields Being JOINed In the previous examples, we specified which field should be used to relate the data in the join (primary/foreign keys): SELECT * FROM a JOIN b USING (id);! This works when the field has the same name If we have multiple fields we need to use, we can specify it as a list of fields, separated by commas SELECT * FROM a JOIN b USING (id, anotherfield);! 21

23 Fields Being JOINed We can also join them on fields that DO NOT have the same name using the keyword ON instead of USING! Example: o Suppose we have a database for a store o The customer table has a field, id o The purchases table has a field, customer, that is a foreign key to the id field in the customer table o Then to join these two tables, we would use: SELECT * FROM customers JOIN purchases ON purchases.customer = customers.id;! o If necessary, multiple clauses can be added: SELECT * FROM customers JOIN purchases ON purchases.customer = customers.id;! 22

24 Fields Being JOINed We can also use a NATURAL JOIN, which will automatically compare any fields that have the same name Example: SELECT * FROM a NATURAL JOIN b;! BUT be careful it may check columns that you don t want compared because they have the same name o EXAMPLE: Suppose you have two tables students: studentwin, name, major Advisors: studentwin, name, department o Doing SELECT * FROM students NATURAL JOIN advisors; will NOT work because it will not only compare the studentwin field, but also the name field 23

25 Cross Product A cross product of two sets is a combination of every item in each set o Example: A = {1,2,3} B = {4,5,6} o Then the cross product of A and B would be: {(1,4), (1,5), (1,6), (2,4), (2,5), (2,6), (3,4), (3,5), (3,6)} Likewise, we can do something similar in MySQL o SELECT * FROM a, b; will return a record for each combination of a record from a and a record from b o We can then simulate a join by doing a comparison in the WHERE clause on the condition that makes the JOIN valid: SELECT * FROM a, b WHERE a.id = b.id;! o However, performance-wise, this may not be as fast as doing a JOIN! 24

26 Relating Tables EXAMPLE: Suppose you have two tables o t1: id, firstname o t2: id, lastname Then these four SQL statements are equivalent in terms of results: o SELECT * FROM t1 NATURAL JOIN t2;! o SELECT * FROM t1 JOIN t2 USING (id);! o SELECT * FROM t1 JOIN t2 ON t1.id = t2.id;! o SELECT * FROM t1, t2 WHERE t1.id = t2.id;! 25

27 Relating Tables LEFT JOIN and RIGHT JOIN both work in relation to the order of the tables as they appear in the SQL statement LEFT JOIN includes all of the records from the table on the left; RIGHT JOIN includes all of the records from the table on the right These two statements are equivalent: o SELECT * FROM a LEFT JOIN b USING (id);! o SELECT * FROM b RIGHT JOIN a USING (id);! 26