Sql1 - Introduction to SQL SelectFromWhere Statements Meaning of queries Subqueries 1 Why SQL x SQL is a veryhighlevel language in which the

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Unformatted text preview: Introduction to SQL SelectFromWhere Statements Meaning of queries Subqueries 1 Why SQL? x SQL is a veryhighlevel language, in which the programmer is able to avoid specifying a lot of datamanipulation details that would be necessary in languages like C++. x What makes SQL viable is that its queries are "optimized" quite well, yielding efficient query executions. 2 SelectFromWhere Statements x The principal form of a query is: 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 x To implement this algorithm think of a tuple variable ranging over 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. 8 * 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'; 9 Result of Query: name `Bud' `Bud Lite' `Michelob' manf `AnheuserBusch' `AnheuserBusch' `AnheuserBusch' Now, the result has each of the attributes of Beers. 10 Renaming Attributes x If you want the result to have different attribute names, use "AS <new name>" to rename an attribute. x Example based on Beers(name, manf): SELECT name AS beer, manf FROM Beers WHERE manf = `Anheuser-Busch' 11 Result of Query: beer `Bud' `Bud Lite' `Michelob' manf `AnheuserBusch' `AnheuserBusch' `AnheuserBusch' 12 Expressions in SELECT Clauses x Any expression that makes sense can appear as an element of a SELECT clause. x Example: from Sells(bar, beer, price): SELECT bar, beer, price * 120 AS priceInYen FROM Sells; 13 Result of Query bar Joe's Sue's ... beer priceInYen Bud 300 Miller 360 ... ... 14 Another Example: Constant Expressions x From Likes(drinker, beer): SELECT drinker, `likes Bud' AS whoLikesBud FROM Likes WHERE beer = `Bud'; 15 Result of Query drinker `Sally' `Fred' ... whoLikesBud `likes Bud' `likes Bud' ... 16 Complex Conditions in WHERE Clause x From 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'; 17 Important Points x Two single quotes inside a string represent the singlequote (apostrophe). x Conditions in the WHERE clause can use AND, OR, NOT, and parentheses in the usual way boolean conditions are built. x SQL is caseinsensitive. In general, upper and lower case characters are the 18 same, except inside quoted strings. Patterns x WHERE clauses can have conditions in which a string is compared with a pattern, to see if it matches. x General form: <Attribute> LIKE <pattern> or <Attribute> NOT LIKE <pattern> x Pattern is a quoted string with % = "any string"; _ = "any character." 19 Example x From Drinkers(name, addr, phone) find the drinkers with exchange 555: SELECT name FROM Drinkers WHERE phone LIKE `%555-_ _ _ _'; 20 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. 21 Comparing NULL's to Values x The logic of conditions in SQL is really 3 valued logic: TRUE, FALSE, UNKNOWN. x When any value is compared with NULL, the truth value is UNKNOWN. x But a query only produces a tuple in the answer if its truth value for the WHERE clause is TRUE (not FALSE or UNKNOWN). 22 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, ) = . 23 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 24 Reason: 2Valued Laws != 3Valued Laws x Some common laws, like the commutativity of AND, hold in 3valued logic. x But others do not; example: the "law of excluded middle," p OR NOT p = TRUE. When p = UNKNOWN, the left side is MAX( , (1 )) = != 1. 25 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>" 26 Example 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; 27 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. 28 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. 29 Example drinker bar tv1 Sally Joe's drinker beer Sally Bud tv2 Frequents check for Joe check these are equal Likes to output 30 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. 31 Example 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; 32 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 place another query, and then query its result. Better use a tuplevariable to name tuples of the result. 33 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. Also typically, a single tuple is guaranteed by keyness of attributes. A runtime error occurs if there is no tuple or more than one tuple. 34 Example x From 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. 35 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'); 36 The IN Operator x <tuple> IN <relation> is true if and only if the tuple is a member of the relation. x INexpressions can appear in WHERE clauses. x The <relation> is often a subquery. 37 <tuple> NOT IN <relation> means the opposite. Example x From 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 FROM Likes The set of beers Fred WHERE drinker = `Fred'); likes 38 The Exists Operator x EXISTS( <relation> ) is true if and only if the <relation> is not empty. x Being a booleanvalued operator, EXISTS can appear in WHERE clauses. x Example: From Beers(name, manf), find those beers that are the unique beer by their manufacturer. 39 Example Query with 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 40 The Operator ANY x x = ANY( <relation> ) is a boolean condition meaning that x equals at least one tuple in the relation. x Similarly, = can be replaced by any of the comparison operators. x Example: x >= ANY( <relation> ) means x is not smaller than all tuples in the relation. Note tuples must have one component only. 41 The Operator ALL x Similarly, x <> ALL( <relation> ) is true if and only if for every tuple t in the relation, x is not equal to t. x The <> can be replaced by any comparison operator. x Example: x >= ALL( <relation> ) means there is no tuple larger than x in the relation. 42 That is, x is not a member of the relation. Example 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); 43 ...
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This note was uploaded on 08/01/2008 for the course CSC 540 taught by Professor Chirkova during the Spring '08 term at N.C. State.

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Sql1 - Introduction to SQL SelectFromWhere Statements Meaning of queries Subqueries 1 Why SQL x SQL is a veryhighlevel language in which the

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