java barcode printing library (Q29) Count the number of di erent sailor names SELECT COUNT ( DISTINCT Ssname ) FROM Sailors S in Software

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(Q29) Count the number of di erent sailor names SELECT COUNT ( DISTINCT Ssname ) FROM Sailors S
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On instance S3, the answer to Q28 is 10, whereas the answer to Q29 is 9 (because two sailors have the same name, Horatio) If DISTINCT is omitted, the answer to Q29 is 10, because the name Horatio is counted twice Thus, without DISTINCT Q29 is equivalent to Q28 However, the use of COUNT (*) is better querying style when it is applicable Aggregate operations o er an alternative to the ANY and ALL constructs For example, consider the following query: (Q30) Find the names of sailors who are older than the oldest sailor with a rating of 10 SELECT Ssname FROM Sailors S WHERE Sage > ( SELECT MAX ( S2age ) FROM Sailors S2 WHERE S2rating = 10 ) On instance S3, the oldest sailor with rating 10 is sailor 58, whose age is 35 The names of older sailors are Bob, Dustin, Horatio, and Lubber Using ALL, this query could alternatively be written as follows: SELECT Ssname FROM Sailors S WHERE Sage > ALL ( SELECT S2age FROM Sailors S2 WHERE S2rating = 10 ) However, the ALL query is more error prone one could easily (and incorrectly!) use ANY instead of ALL, and retrieve sailors who are older than some sailor with a rating of 10 The use of ANY intuitively corresponds to the use of MIN, instead of MAX, in the previous query
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551 The GROUP BY and HAVING Clauses
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Thus far, we have applied aggregate operations to all (qualifying) rows in a relation Often we want to apply aggregate operations to each of a number of groups of rows in a relation, where the number of groups depends on the relation instance (ie, is not known in advance) For example, consider the following query
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SQL: Queries, Programming, Triggers
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(Q31) Find the age of the youngest sailor for each rating level
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If we know that ratings are integers in the range 1 to 10, we could write 10 queries of the form: SELECT MIN (Sage) FROM Sailors S WHERE Srating = i where i = 1, 2, , 10 Writing 10 such queries is tedious More importantly, we may not know what rating levels exist in advance To write such queries, we need a major extension to the basic SQL query form, namely, the GROUP BY clause In fact, the extension also includes an optional HAVING clause that can be used to specify quali cations over groups (for example, we may only be interested in rating levels > 6) The general form of an SQL query with these extensions is: SELECT FROM WHERE GROUP BY HAVING [ DISTINCT ] select-list from-list quali cation grouping-list group-quali cation
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Using the GROUP BY clause, we can write Q31 as follows: SELECT Srating, MIN (Sage) FROM Sailors S GROUP BY Srating Let us consider some important points concerning the new clauses: The select-list in the SELECT clause consists of (1) a list of column names and (2) a list of terms having the form aggop ( column-name ) AS new-name The optional AS new-name term gives this column a name in the table that is the result of the query Any of the aggregation operators can be used for aggop Every column that appears in (1) must also appear in grouping-list The reason is that each row in the result of the query corresponds to one group, which is a collection of rows that agree on the values of columns in grouping-list If a column appears in list (1), but not in grouping-list, it is not clear what value should be assigned to it in an answer row The expressions appearing in the group-quali cation in the HAVING clause must have a single value per group The intuition is that the HAVING clause determines
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