Review: Where have we been?

Transcription

1 SQL Basic

2 Review Query languages provide 2 key advantages: Less work for user asking query More opportunities for optimization Algebra and safe calculus are simple and powerful models for query languages for relational model Have same expressive power Algebra is more operational; calculus is more declarative SQL can express every query that is expressible in relational algebra/calculus. (and more)

3 Review: Where have we been? Theory Relational Calculus Lecture 6 Practice Query Optimization and Execution Relational Algebra Relational Model Lecture 5 Lectures 3 &4 Lecture 2 Relational Operators Files and Access Methods Buffer Management Disk Space Management DB

4 Where are we going next? SQL This week After the midterm Next week Practice Query Optimization and Execution Relational Operators Files and Access Methods Buffer Management Disk Space Management DB

5 Review: Relational Calculus Example Find names, ages and reservation dates of sailors rated > 7 who ve reserved boat #103 3 quantifiers, but only 1 is free. The free quantifier defines the shape of the result. {S1 S Sailors S.rating > 7 R(R Reserves R.bid = 103 R.sid = S.sid) (S1.sname = S.name S1.age = S.age S1.day = R.day)} S S S R R sid sname rating age 22 dustin lubber rusty sid bid day /10/ /12/96 sname age day S1 rusty /12/96

6 Review: The SQL Query Language The most widely used relational query language. Standardized (although most systems add their own special sauce -- including PostgreSQL) We will study SQL92 -- a basic subset

7 Review: SQL Two sublanguages: DDL Data Definition Language Define and modify schema (at all 3 levels) DML Data Manipulation Language Queries and IUD (insert update delete) DBMS is responsible for efficient evaluation. Relational completeness means we can define precise semantics for relational queries. Optimizer can re-order operations, without affecting query answer. Choices driven by cost model

8 Review: DDL CREATE TABLE Sailors (sid INTEGER, NOT NULL, sname CHAR(20), rating INTEGER, age REAL, PRIMARY KEY sid) CREATE TABLE Boats (bid INTEGER, bname CHAR NOT (20), NULL, color CHAR(10) PRIMARY KEY bid) CREATE TABLE Reserves (sid INTEGER, bid INTEGER, NOT NULL, day DATE, NOT NULL, PRIMARY KEY NOT (sid, NULL, bid, day), FOREIGN KEY sid REFERENCES Sailors, FOREIGN KEY bid REFERENCES Boats) Sailors sid sname rating age 1 Frodo Bilbo Sam 8 27 Boats bid bname color 101 Nina red 102 Pinta blue 103 Santa Maria red Reserves sid bid day / /13

9 Integrity Constraints (ICs) A foreign key constraint is an Integrity Constraint: a condition that must be true for any instance of the database; Specified when schema is defined. Checked when relations are modified. Primary/foreign key constraints; but databases support more general constraints as well. e.g. domain constraints like: Rating must be between 1 and 10 ALTER TABLE SAILORS ADD CONSTRAINT RATING CHECK (RATING >= 1 AND RATING < 10) Or even more complex (and potentially nonsensical): ALTER TABLE SAILORS ADD CONSTRAINT RATING CHECK (RATING*AGE/4 <= SID)

10 DBMSs have fairly sophisticated support for constraints! Specify them on CREATE or ALTER TABLE statements Column Constraints: expressions for column constraint must produce boolean results and reference the related column s value only. NOT NULL NULL UNIQUE PRIMARY KEY CHECK (expression) FOREIGN KEY (column) referenced_table [ ON DELETE action ] [ ON UPDATE action ] } action is one of: NO ACTION, CASCADE, SET NULL, SET DEFAULT

11 DBMSs have fairly sophisticated support for constraints! Table Constraints: UNIQUE ( column_name [,... ] ) PRIMARY KEY ( column_name [,... ] ) CHECK ( expression ) FOREIGN KEY ( column_name [,... ] ) REFERENCES reftable [ ON DELETE action ] [ ON UPDATE action ] } Here, expressions, keys, etc can include multiple columns

12 Integrity Constraints can help prevent data consistency errors but they have drawbacks: Expensive Can t always return a meaningful error back to the application. e.g: What if you saw this error when you enrolled in a course online? A violation of the constraint imposed by a unique index or a unique constraint occurred. Can be inconvenient e.g. What if the Sailing Class application wants to register new (unrated) sailors with rating 0? So they aren t widely used Software developers often prefer to keep the integrity logic in applications instead

13 Intermission

14 SQL DML DML includes 4 main statements: SELECT (query), INSERT, UPDATE and DELETE We ll spend a lot of time on this one e.g: To find the names of all 19 year old students: PROJECT SELECT S.name FROM Students S WHERE S.age=19 SELECT sid name login age gpa Jones jones@cs Smith smith@ee Smith smith@math

15 Querying Multiple Relations Can specify a join over two tables as follows: SELECT S.name, E.cid PROJECT FROM Students S, Enrolled E WHERE S.sid=E.sid AND E.grade= B' JOIN sid cid grade Carnatic101 C Reggae203 B Topology112 A History105 B SELECT sid name login age gpa Jones jones@cs Smith smith@ee result = S.name Jones E.cid History105

16 Basic SQL Query DISTINCT: optional keyword indicating answer should not contain duplicates. In SQL, default is that duplicates are not eliminated! (Result is called a multiset ) SELECT FROM WHERE target-list : A list of attributes of tables in relation-list [DISTINCT] target-list relation-list qualification qualification : Comparisons combined using AND, OR and NOT. Comparisons are Attr op const or Attr1 op Attr2, where op is one of,,,, etc. relation-list : A list of relation names, possibly with a rangevariable after each name

17 Query Semantics Semantics of an SQL query are defined in terms of the following conceptual evaluation strategy: 1. FROM clause: compute cross-product of all tables 2. WHERE clause: Check conditions, discard tuples that fail. (called selection ). 3. SELECT clause: Delete unwanted fields. (called projection ). 4.If DISTINCT specified, eliminate duplicate rows. Probably the least efficient way to compute a query! An optimizer will find more efficient strategies to get the same answer.

18 Query Semantics Example SELECT sname FROM Sailors, Reserves WHERE Sailors.sid=Reserves.sid AND bid=103 Boats Sailors sid sname rating age 1 Frodo Bilbo Sam 8 27 bid bname color X 101 Nina red 102 Pinta blue 103 Santa Maria red Reserves sid bid day / /13

19 Step 1: Compute the cross product Sailors Reserves sid sname rating age 1 Frodo Bilbo Sam 8 27 SailorsXReserves... sid bid day / /13 sid sname rating age sid bid day 1 Frodo /12 1 Frodo /13 2 Bilbo /12 2 Bilbo /13 3 Sam /12 3 Sam /13

20 Step 1: How big? Sailors sid sname rating age 1 Frodo Bilbo Sam 8 27 Reserves sid bid day / /13 Question: If S is cardinality of Sailors, and R is cardinality of Reserves, What is the cardinality of Sailors X Reserves? Answer: S * R Sailors X Reserves = 3X2 = 6

21 Step 2: Check conditions in where clause SELECT sname FROM Sailors, Reserves WHERE Sailors.sid=Reserves.sid AND bid=103 SailorsXReserves sid sname rating age sid bid day 1 Frodo /12 1 Frodo /13 2 Bilbo /12 2 Bilbo /13 3 Sam /12 3 Sam /13

22 Step 3: Delete unwanted fields SELECT sname FROM Sailors, Reserves WHERE Sailors.sid=Reserves.sid AND bid=103 SailorsXReserves sid sname rating age sid bid day 1 Frodo /12 1 Frodo /13 2 Bilbo /12 2 Bilbo /13 3 Sam /12 3 Sam /13

23 Range Variables Used for short hand Needed when ambiguity could arise e.g two tables with the same column name: SELECT sname FROM Sailors, Reserves WHERE Sailors.sid=Reserves.sid AND Reserves.bid=103 SELECT sname FROM Sailors S, Reserves R WHERE S.sid=R.sid AND R.bid=103 Question: do range variables remind you of anything? Variables in relational calculus

24 Sometimes you need a range variable e.g a Self-join: SELECT R1.bid, R1.date FROM Reserves R1, Reserves R2 WHERE R1.bid = R2.bid and R1.date = R2.date and R1.sid!= R2.sid Reserves Reserves bid day 103 9/ /12 R1 R1 R1 sid bid day / / / /12 R2 R2 R2 R2 sid bid day / / / /12

25 Sometimes you need a range variable SELECT R1.bid, R1.day FROM Reserves R1, Reserves R2 WHERE R1.bid = R2.bid and R1.day = R2.day and R1.sid!= R2.sid bid day 103 9/ /12 What are we computing? Boats reserved on the same day by different sailors

26 SELECT Clause Expressions Can use * if you want all columns: SELECT * FROM Sailors x WHERE x.age > 20 Can use arithmetic expressions (add other operations we ll discuss later) SELECT S.age, S.age-5 AS age1, 2*S.age AS age2 FROM Sailors S WHERE S.sname = Dustin Can use AS to provide column names SELECT S1.sname AS name1, S2.sname AS name2 FROM Sailors S1, Sailors S2 WHERE 2*S1.rating = S2.rating - 1

27 WHERE Clause Expressions Can also have expressions in WHERE clause: SELECT S1.sname AS name1, S2.sname AS name2 FROM Sailors S1, Sailors S2 WHERE 2*S1.rating = S2.rating - 1 LIKE is used for string matching. SELECT S.age, S.age-5 AS age1, 2*S.age AS age2 FROM Sailors S WHERE S.sname LIKE B_l%o `_ stands for any one character and `% stands for 0 or more arbitrary characters.

28 SELECT DISTINCT Sailors sid sname rating age 1 Frodo Bilbo Sam 8 27 Reserves sid bid day / / /13 SELECT DISTINCT S.sid FROM Sailors S, Reserves R WHERE S.sid=R.sid Find sailors that have reserved at least one boat 2 sid 1

29 SELECT DISTINCT How about: SELECT S.sid FROM Sailors S, Reserves R WHERE S.sid=R.sid sid 1 2 2

30 SELECT DISTINCT How about: SELECT S.sname FROM Sailors S, Reserves R WHERE S.sid=R.sid vs: SELECT DISTINCT S.sname FROM Sailors S, Reserves R WHERE S.sid=R.sid sname Frodo Bilbo Bilbo sname Frodo Bilbo Sailors sid sname rating age 1 Frodo Bilbo Sam Bilbo 5 32 Reserves sid bid day / / /13 Do we find all sailors that reserved at least one boat?

31 ANDs, ORs, UNIONs and INTERSECTs Find sids of sailors who ve reserved a red or a green boat SELECT R.sid FROM Boats B,Reserves R WHERE(B.color= red OR B.color= green ) AND R.bid=B.bid Sailors sid sname rating age X 1 Frodo Bilbo Sam 8 27 sid 2 4 Boats bid bname color 101 Nina red 102 Pinta blue 103 Santa Maria red 105 Titanic green Reserves sid bid day / / /13

32 ANDs and ORs Find sids of sailors who ve reserved a red and a green boat Boats SELECT R.sid FROM Boats X B,Reserves R WHERE(B.color= red AND B.color= green ) AND R.bid=B.bid Sailors sid sname rating age 1 Frodo Bilbo Sam 8 27 bid bname color 101 Nina red 102 Pinta blue 103 Santa Maria red 105 Titanic green Reserves sid bid day / / /13

33 Use INTERSECT instead of AND SELECT R.sid FROM Boats B,Reserves R WHERE B.color = red AND R.bid=B.bid INTERSECT SELECT R.sid FROM Boats B,Reserves R WHERE B.color = green AND R.bid=B.bid sid 1 2 sid 1 = sid 1 Exercise: try to rewrite this query using a self join instead of INTERSECT! Boats bid bname color 101 Nina red 102 Pinta blue 103 Santa Maria red 105 Titanic green Reserves sid bid day / / /13

34 Could also use UNION for the OR query SELECT R.sid FROM Boats B, Reserves R WHERE B.color = red AND R.bid=B.bid UNION SELECT R.sid FROM Boats B, Reserves R WHERE B.color = green AND R.bid=B.bid sid 2 sid 4 = sid 2 4 Boats bid bname color 101 Nina red 102 Pinta blue 103 Santa Maria red 105 Titanic green Reserves sid bid day / / /13

35 EXCEPT: Set Difference Find sids of sailors who have not reserved a boat SELECT S.sid FROM Sailors S EXCEPT SELECT S.sid FROM Sailors S, Reserves R WHERE S.sid=R.sid First find the set of sailors who have reserved a boat and then compare it with the rest of the sailors Sailors sid sname rating age 1 Frodo Bilbo Sam 8 27 Reserves sid bid day / / /13 sid 3