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SQL advanced select using Oracle

SQL advanced select using Oracle. 7. Multiple Tables: Joins and Set Operations 8. Subqueries: Nested Queries. Select, 156. Simple data from a single table Advanced data from more tables join sub-queries. Join, 157. Cartesian product select * from tableA, tableB

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SQL advanced select using Oracle

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  1. SQL advanced select using Oracle 7. Multiple Tables: Joins and Set Operations 8. Subqueries: Nested Queries SQL advanced select using Oracle

  2. Select, 156 • Simple • data from a single table • Advanced • data from more tables • join • sub-queries SQL advanced select using Oracle

  3. Join, 157 • Cartesian product • select * from tableA, tableB • all rows in tableA combined with all rows from tableB • huge result! seldom useful!! • Different types of joins • equi-join • non-equi-join • outer join • self-join SQL advanced select using Oracle

  4. Equi-join, 158 • Joining 2 tables using common attributes [usually primary and foreign keys] • Structure select … from tableA, tableB where tableA.PK = tableB.FK • Comparison using equality (=) is equi-join • Examples select * from student, city where city.postcode = student.postcode select * from student, course, studentCourse where student.ID = studentCourse.studentID and studentCourse.courseID = course.ID SQL advanced select using Oracle

  5. Non-equi-joins, 161 • Where clause using operations other than equality • Fig. 7-6, page 162 SQL advanced select using Oracle

  6. Outer joins, 163 • Ordinary join • if a value in one table has no matching value in the other table, the row is not included in the result. • Outer join • values without matching values are included in the result. • Oracle has 2 syntaxes • Special syntax • Special (old) Oracle syntax • New syntax • SQL99 standard syntax SQL advanced select using Oracle

  7. Outer join, old syntax • Old / special Oracle syntax SELECT … FROM tableA, tableB WHERE tableA.col1 (+)= tableB.col2 • rows from tableA with no matching attribute in tableB is included in the result • Fig. 7-7, page 163 SQL advanced select using Oracle

  8. Outer join, new syntax • New / SQL 99 standard syntax SELECT d.department_id, e.last_name FROM departments d LEFT OUTER JOIN employees e ON d.department_id = e.department_id ORDER BY d.department_id • Result • Department with and without employees are included in the result • New in Oracle 9i (not present in Oracle 8i) • No examples in the book! • Look in the Oracle SQL documentation for more examples SQL advanced select using Oracle

  9. Self-join, 165 • Joining a table with itself. • ER recursive relationship • Rel. model foreign key to another attribute in the same relation. • same table used twice in the from clause we need to name the two instances of the table (table aliases) • select … from employee e, employee s where e.supervisorID = s.ID • Fig. 7-9 page 165 SQL advanced select using Oracle

  10. Set operators, 166 • Tables may be used in set operations • union tableA union tableB • all rows in tableA and all rows in tableB • union all tableA union all tableB • as union, but includes duplicates • intersect tableA intersect tableB • all rows that appears in both tableA and tableB • minus tableA minus tableB • all rows in tableA, but not in tableB • Tables must be "union compatible" • same number + types + order of attributes SQL advanced select using Oracle

  11. Sub-query, 174 • The where clause in a select statement may contain another select statement (sub-query)! • Fig. 8-1, page 175 • Two types of sub-queries • single row • returning a single row of data, and most often a single data item • multiple row • returning more than on row of data SQL advanced select using Oracle

  12. Single row query, 174 • Inner query is executed first, returning a value that is used in the outer query. • select … from tableA where attributA operator (sub-query) • Operator may be: =, >, <, etc. SQL advanced select using Oracle

  13. Sub-queries in create, insert, update and delete, 176 • create table tableName as select query • pure redundancy! • insert into tableName select query • pure redundancy! • update tableName set (columnNames) = (query) where condition • delete from tableName where columnName = (query) SQL advanced select using Oracle

  14. Multiple-row sub-queries, 181 • Sub-query returning more than one row of data. • select … from … where attribute operator (sub-query) • Operator may be • in • <all, >all • <any, >any, =any • Figure 8-11, 8-12, page 182 • Figure 8-13, page 183 SQL advanced select using Oracle

  15. Top-N analysis,183 • MS Access SQL and SQL Server has a special keyword top • select top 10 attribute … • Oracle has no such feature, but it can be simulated using the pseudo-column rownum. • Fig. 8-14 + 8-15, page 184 • sub-query in from clause • inline view [more on views later on] SQL advanced select using Oracle

  16. Correlated subquery,185 • Ordinary subquery • The inner query does not reference columns from the outer query • The inner query can be “calculated” once and for all. • Correlated subquery • The inner query references columns from the outer query • Like programming 2 loops, one inside the other • Figure 8-17, page 187 SQL advanced select using Oracle

  17. The EXISTS operator, 187 • New operator used with correlated sub-queries • Result • False • If the current result of the sub-query is empty • True • If the current result of the sub-query is not empty • Examples • Figure 8-18, page 187 • At least one student has the faculty (teacher) • Select ‘1’ • We have to select something, but in this case we don’t use it! • Figure 8-19, page 188 • No student has the faculty (teacher) SQL advanced select using Oracle

  18. Final remarks • SQL (specially select) is like programming. • There's only one place to learn it - at the keyboard! • Make advanced select statements incrementally • Starting from the innermost sub-query. • Monitor the results • Take advantage of advanced select statements • Don't try to program the same features in your application. SQL advanced select using Oracle

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