Join Queries A join query is (Free web hosting services) a query
Join Queries A join query is a query retrieving records from more than one table. Records from different tables are usually joined on related key field values. The most efficient and effective forms of join are those between directly related primary and foreign key fields. There are a number of different types of joins: . Inner Join An intersection between two tables using matching field values, returning records common to both tables only. Inner join syntax is as follows: SELECT … FROM table [alias] [, … ] [ INNER JOIN table [alias] [ USING (field [, … ]) | ON (field = field [{AND | OR} [NOT] [ … ]) ] ] [ WHERE … ] [ GROUP BY … ] [ ORDER BY … ]; The following query is an inner join because it finds all publishers and related published editions. The two tables are linked based on the established primary key to foreign key relationship. The primary key is in the PUBLISHER table on the one side of the one-to-many relationship, between the PUBLISHER and EDITION tables. The foreign key is precisely where it should be, on the many side of the one-to-many relationship. SELECT P.NAME AS PUBLISHER, E.ISBN FROM PUBLISHER P JOIN EDITION E USING (PUBLISHER_ID); PUBLISHER ISBN ——————————– ———- Overlook Press 1585670081 Ballantine Books 345333926 Ballantine Books 345336275 Ballantine Books 345438353 Bantam Books 553293362 Spectra 553278398 Spectra 553293370 Spectra 553293389 Oxford University Press 198711905 L P Books 893402095 Del Rey Books 345308999 Del Rey Books 345334787 Del Rey Books 345323440 Books on Tape 5553673224 Books on Tape 5557076654 A common programming error is to get the purpose of the WHERE and HAVING clause filters mixed up. The WHERE clause filters records as they are read (as I/O activity takes place) from the database. The HAVING clause filters aggregated groups, after all database I/O activity has completed. Don t use the HAVING clause when the WHERE clause should be used, and visa versa. 137 Reading and Writing Data with SQL
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