SQL advanced select using Oracle 1 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 –uniontableA union tableB all rows in tableA and all rows in tableB –union alltableA union all tableB as union, but includes duplicates –intersecttableA intersect tableB all rows that appears in both tableA and tableB –minustableA 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 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 , 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.