sql1 - Introduction to SQL SelectFromWhere Statements...

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Unformatted text preview: Introduction to SQL SelectFromWhere Statements Multirelation Queries Subqueries 1 Why SQL? x SQL is a veryhighlevel language. Say "what to do" rather than "how to do it." Avoid a lot of datamanipulation details needed in procedural languages like C++ or Java. x Database management system figures out "best" way to execute query. Called "query optimization." 2 SelectFromWhere Statements SELECT desired attributes FROM one or more tables WHERE condition about tuples of the tables 3 Our Running Example x All our SQL queries will be based on the following database schema. Underline indicates key attributes. Beers(name, manf) Bars(name, addr, license) Drinkers(name, addr, phone) Likes(drinker, beer) Sells(bar, beer, price) Frequents(drinker, bar) 4 Example x Using Beers(name, manf), what beers are made by AnheuserBusch? SELECT name FROM Beers WHERE manf = 'Anheuser-Busch'; 5 Result of Query name Bud Bud Lite Michelob . . . The answer is a relation with a single attribute, name, and tuples with the name of each beer by AnheuserBusch, such as Bud. 6 Meaning of SingleRelation Query x Begin with the relation in the FROM clause. x Apply the selection indicated by the WHERE clause. x Apply the extended projection indicated by the SELECT clause. 7 Operational Semantics name manf Bud AnheuserBusch Include t.name in the result, if so Check if AnheuserBusch Tuplevariable t loops over all tuples 8 Operational Semantics General x Think of a tuple variable visiting each tuple of the relation mentioned in FROM. x Check if the "current" tuple satisfies the WHERE clause. x If so, compute the attributes or expressions of the SELECT clause using the components of this tuple. 9 * In SELECT clauses x When there is one relation in the FROM clause, * in the SELECT clause stands for "all attributes of this relation." x Example: Using Beers(name, manf): SELECT * FROM Beers WHERE manf = 'Anheuser-Busch'; 10 Result of Query: name manf Bud AnheuserBusch Bud Lite AnheuserBusch Michelob AnheuserBusch . . . . . . Now, the result has each of the attributes of Beers. 11 Renaming Attributes x If you want the result to have different attribute names, use "AS <new name>" to rename an attribute. x Example: Using Beers(name, manf): SELECT name AS beer, manf FROM Beers WHERE manf = 'Anheuser-Busch' 12 Result of Query: beer manf Bud AnheuserBusch Bud Lite AnheuserBusch Michelob AnheuserBusch . . . . . . 13 Expressions in SELECT Clauses x Any expression that makes sense can appear as an element of a SELECT clause. x Example: Using Sells(bar, beer, price): SELECT bar, beer, price*114 AS priceInYen FROM Sells; 14 Result of Query bar Joe's Sue's ... beer priceInYen Bud 285 Miller 342 ... ... 15 Example: Constants as Expressions x Using Likes(drinker, beer): SELECT drinker, 'likes Bud' AS whoLikesBud FROM Likes WHERE beer = 'Bud'; 16 Result of Query drinker Sally Fred ... whoLikesBud likes Bud likes Bud ... 17 Example: Information Integration x We often build "data warehouses" from the data at many "sources." x Suppose each bar has its own relation Menu(beer, price) . x To contribute to Sells(bar, beer, price) we need to query each bar and insert the name of the bar. 18 Information Integration (2) x For instance, at Joe's Bar we can issue the query: SELECT 'Joe''s Bar', beer, price FROM Menu; 19 Complex Conditions in WHERE Clause x Boolean operators AND, OR, NOT. x Comparisons =, <>, <, >, <=, >=. And many other operators that produce booleanvalued results. 20 Example: Complex Condition x Using Sells(bar, beer, price), find the price Joe's Bar charges for Bud: SELECT price FROM Sells WHERE bar = 'Joe''s Bar' AND beer = 'Bud'; 21 Patterns x A condition can compare a string to a pattern by: x Pattern is a quoted string with % = "any string"; _ = "any character." <Attribute> LIKE <pattern> or <Attribute> NOT LIKE <pattern> 22 Example: LIKE x Using Drinkers(name, addr, phone) find the drinkers with exchange 555: SELECT name FROM Drinkers WHERE phone LIKE '%555-_ _ _ _'; 23 NULL Values x Tuples in SQL relations can have NULL as a value for one or more components. x Meaning depends on context. Two common cases: Missing value : e.g., we know Joe's Bar has some address, but we don't know what it is. Inapplicable : e.g., the value of attribute spouse for an unmarried person. 24 Comparing NULL's to Values x The logic of conditions in SQL is really 3 valued logic: TRUE, FALSE, UNKNOWN. x Comparing any value (including NULL itself) with NULL yields UNKNOWN. x A tuple is in a query answer iff the WHERE clause is TRUE (not FALSE or UNKNOWN). 25 ThreeValued Logic x To understand how AND, OR, and NOT work in 3valued logic, think of TRUE = 1, FALSE = 0, and UNKNOWN = . x AND = MIN; OR = MAX, NOT(x) = 1x. x Example: TRUE AND (FALSE OR NOT(UNKNOWN)) = MIN(1, MAX(0, (1 ))) = MIN(1, MAX(0, )) = MIN(1, ) = . 26 Surprising Example x From the following Sells relation: bar beer price Joe's Bar Bud NULL SELECT bar FROM Sells WHERE price < 2.00 OR price >= 2.00; UNKNOWN UNKNOWN UNKNOWN 27 Reason: 2Valued Laws != 3Valued Laws x Some common laws, like commutativity of AND, hold in 3valued logic. x But not others, e.g., the law of the excluded middle : p OR NOT p = TRUE. When p = UNKNOWN, the left side is MAX( , (1 )) = != 1. 28 Multirelation Queries x Interesting queries often combine data from more than one relation. x We can address several relations in one query by listing them all in the FROM clause. x Distinguish attributes of the same name by "<relation>.<attribute>" . 29 Example: Joining Two Relations x Using relations Likes(drinker, beer) and Frequents(drinker, bar), find the beers liked by at least one person who frequents Joe's Bar. SELECT beer FROM Likes, Frequents WHERE bar = 'Joe''s Bar' AND Frequents.drinker = Likes.drinker; 30 Formal Semantics x Almost the same as for singlerelation queries: 1. Start with the product of all the relations in the FROM clause. 2. Apply the selection condition from the WHERE clause. 3. Project onto the list of attributes and expressions in the SELECT clause. 31 Operational Semantics x Imagine one tuplevariable for each relation in the FROM clause. These tuplevariables visit each combination of tuples, one from each relation. x If the tuplevariables are pointing to tuples that satisfy the WHERE clause, send these tuples to the SELECT clause. 32 Example drinker bar tv1 Sally Joe's drinker beer Sally Bud tv2 Frequents check for Joe check these are equal Likes to output 33 Explicit TupleVariables x Sometimes, a query needs to use two copies of the same relation. x Distinguish copies by following the relation name by the name of a tuple variable, in the FROM clause. x It's always an option to rename relations this way, even when not essential. 34 Example: SelfJoin x From Beers(name, manf), find all pairs of beers by the same manufacturer. Do not produce pairs like (Bud, Bud). Produce pairs in alphabetic order, e.g. (Bud, Miller), not (Miller, Bud). SELECT b1.name, b2.name FROM Beers b1, Beers b2 WHERE b1.manf = b2.manf AND b1.name < b2.name; 35 Subqueries x A parenthesized SELECTFROM WHERE statement (subquery ) can be used as a value in a number of places, including FROM and WHERE clauses. x Example: in place of a relation in the FROM clause, we can use a subquery and then query its result. Must use a tuplevariable to name tuples of the result. 36 Example: Subquery in FROM x Find the beers liked by at least one person who frequents Joe's Bar. Drinkers who frequent Joe's Bar SELECT beer FROM Likes, (SELECT drinker FROM Frequents WHERE bar = 'Joe''s Bar')JD WHERE Likes.drinker = JD.drinker; 37 Subqueries That Return One Tuple x If a subquery is guaranteed to produce one tuple, then the subquery can be used as a value. Usually, the tuple has one component. A runtime error occurs if there is no tuple or more than one tuple. 38 Example: SingleTuple Subquery x Using Sells(bar, beer, price), find the bars that serve Miller for the same price Joe charges for Bud. x Two queries would surely work: 1. Find the price Joe charges for Bud. 2. Find the bars that serve Miller at that price. 39 Query + Subquery Solution SELECT bar FROM Sells WHERE beer = 'Miller' AND price = (SELECT price FROM Sells The price at which Joe WHERE bar = 'Joe''s Bar' sells Bud AND beer = 'Bud'); 40 The IN Operator x <tuple> IN (<subquery>) is true if and only if the tuple is a member of the relation produced by the subquery. x INexpressions can appear in WHERE clauses. Opposite: <tuple> NOT IN (<subquery>). 41 Example: IN x Using Beers(name, manf) and Likes(drinker, beer), find the name and manufacturer of each beer that Fred likes. SELECT * FROM Beers WHERE name IN (SELECT beer The set of FROM Likes beers Fred likes WHERE drinker = 'Fred'); 42 Remember These From Lecture #1? SELECT a FROM R, S WHERE R.b = S.b; SELECT a FROM R WHERE b IN (SELECT b FROM S); 43 IN is a Predicate About R's Tuples SELECT a Two 2's FROM R WHERE b IN (SELECT b FROM S); a b 1 2 3 4 R b c 2 5 2 6 S (1,2) satisfies the condition; 1 is output once. One loop, over the tuples of R 44 This Query Pairs Tuples from R, S SELECT a FROM R, S WHERE R.b = S.b; a b 1 2 3 4 R b c 2 5 2 6 S (1,2) with (2,5) and (1,2) with (2,6) both satisfy the condition; 1 is output twice. 45 Double loop, over the tuples of R and S The Exists Operator x EXISTS(<subquery>) is true if and only if the subquery result is not empty. x Example: From Beers(name, manf) , find those beers that are the unique beer by their manufacturer. 46 Example: EXISTS Notice scope rule: manf refers SELECT name to closest nested FROM with a relation having that attribute. FROM Beers b1 WHERE NOT EXISTS ( Set of SELECT * Notice the beers FROM Beers SQL "not with the equals" same WHERE manf = b1.manf AND operator manf as b1, but name <> b1.name); not the same beer 47 The Operator ANY x x = ANY(<subquery>) is a boolean condition that is true iff x equals at least one tuple in the subquery result. x Example: x >= ANY(<subquery>) means x is not the uniquely smallest tuple produced by the subquery. Note tuples must have one component only. 48 = could be any comparison operator. The Operator ALL x x <> ALL(<subquery>) is true iff for every tuple t in the relation, x is not equal to t. x <> can be any comparison operator. x Example: x >= ALL(<subquery>) means there is no tuple larger than x in the subquery result. 49 That is, x is not in the subquery result. Example: ALL x From Sells(bar, beer, price), find the beer(s) sold for the highest price. SELECT beer price from the outer FROM Sells Sells must not be less than any price. WHERE price >= ALL( SELECT price FROM Sells); 50 Union, Intersection, and Difference x Union, intersection, and difference of relations are expressed by the following forms, each involving subqueries: (<subquery>) UNION (<subquery>) (<subquery>) INTERSECT (<subquery>) (<subquery>) EXCEPT (<subquery>) 51 Example: Intersection x Using Likes(drinker, beer), Sells(bar, beer, price), and Frequents(drinker, bar), find the drinkers and beers such that: 1. The drinker likes the beer, and 2. The drinker frequents at least one bar that sells the beer. 52 Notice trick: subquery is really a stored table. Solution (SELECT * FROM Likes) INTERSECT (SELECT drinker, beer FROM Sells, Frequents WHERE Frequents.bar = Sells.bar ); The drinker frequents a bar that sells the beer. 53 Bag Semantics x Although the SELECTFROMWHERE statement uses bag semantics, the default for union, intersection, and difference is set semantics. That is, duplicates are eliminated as the operation is applied. 54 Motivation: Efficiency x When doing projection, it is easier to avoid eliminating duplicates. x For intersection or difference, it is most efficient to sort the relations first. Just work tupleatatime. At that point you may as well eliminate the duplicates anyway. 55 Controlling Duplicate Elimination x Force the result to be a set by SELECT DISTINCT . . . x Force the result to be a bag (i.e., don't eliminate duplicates) by ALL, as in . . . UNION ALL . . . 56 Example: DISTINCT x From Sells(bar, beer, price), find all the different prices charged for beers: SELECT DISTINCT price FROM Sells; x Notice that without DISTINCT, each price would be listed as many times as there were bar/beer pairs at that price. 57 Example: ALL x Using relations Frequents(drinker, bar) and Likes(drinker, beer): (SELECT drinker FROM Frequents) EXCEPT ALL (SELECT drinker FROM Likes); x Lists drinkers who frequent more bars than they like beers, and does so as many times as the difference of those counts. 58 Join Expressions x SQL provides several versions of (bag) joins. x These expressions can be standalone queries or used in place of relations in a FROM clause. 59 Products and Natural Joins x Natural join: R NATURAL JOIN S; x Product: R CROSS JOIN S; x Example: Likes NATURAL JOIN Sells; x Relations can be parenthesized subqueries, as well. 60 Theta Join x R JOIN S ON <condition> x Example: using Drinkers(name, addr) and Frequents(drinker, bar): Drinkers JOIN Frequents ON name = drinker; gives us all (d, a, d, b) quadruples such that drinker d lives at address a and frequents bar b. 61 ...
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sql1 - Introduction to SQL SelectFromWhere Statements...

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