Oracle’s take on joins Where it differs from ANSI standard

ANSI Standard SQL join Select supplier_name, Stock_description from supplier join stock on supplier.supplier_id = stock.supplier_id; This also works in Oracle 10g, but Oracle’s native method is…

Oracle SQL join Select supplier_name, Stock_description from supplier, stock where supplier.supplier_id = stock.supplier_id; If you followed the relational algebra discussion in Semester 1, you will know that this is a selection and a projection on a Cartesian product

This makes it easy… To do an inequality join: –Select

Outer join This shows us all the data from one table and anything that joins it from another table: –E.g. all the customers and any orders they have made. –Or All the suppliers and any stock they supply.

Standard outer join Select customer_name, corderno from customer left join corder on customer.customer_id = corder.customer_id; These will work in Oracle, but Oracle prefers:

Oracle outer join Select customer_name, corderno from customer, corder where customer.customer_id(+) = corder.customer_id; –The (+) is on the side that may return nulls.

Exercise Write the query to return all supplier names and the description of the stock they supply, from the builder schema. –Using ‘left join’ –Using (+)

Non-equality joins These concern joins that are NOT necessarily based on referential integrity E.g. –Select all stock for a given supplier (equi-join) where the stock value (i.e. unit_price * stock_level) is less than the amount owed to the supplier (non-equi-join) –Try this in Oracle.

Self-join See the employee example in Tutorial week 11 semester 1, where the employees table has ‘reports to’ field, indicating another row in the same table.

Substitution variables In order to force data entry, a ‘substitution variable’ can be used. This is done by placing an ampersand & in front of the variable name. When SQL*Plus interprets the line, it immediately requests data entry from the user SQL> select * from builder.customer where customer_id = &cid; –SQL* plus will respond by: Enter value for cid: 3

Further uses for substitution variables Substitution variables can be used to replace actual parameters in a procedure call, or even fields: E.g. SQL> Select &fld1,&fld2 from builder.customer; Enter value for fld1: ‘customer_id’ Enter value for fld2: ‘customer_name’

Things to remember When entering values for substitution variables, the same rules apply as when using inserts: –Numerics can be entered without quotes –Strings require single quotes –Dates should be surrounded by single quotes.

Use of ‘ACCEPT’ If you want to change the prompt, you may use the ‘Accept’ command. This command: –Sets up a variable that retains its value for the entire session –Doesn’t use an ampersand (&) –Allows the programmer to design the prompt. E.g. ACCEPT st_code PROMPT: ‘Please enter a 4-character stock code, surrounded by single quotes’;

Script files A script file is an anonymous block. It can be opened and executed, without being saved to the database. Often, you need to run a sequence of SQL statements together, to select, update, insert or delete rows. Sometimes, when a new row is being added, it is necessary to generate a key (e.g. each new student gets a new student number)

Declaring sequences A sequence is an object that is defined in the database and can be incremented whenever a new value (such as a key) is needed. The sequence is created and given a name, an increment, a start value and a possible maximum value. E.g. Create SEQUENCE empIdSeq Incremented by 1 Start with 100 Maxvalue 400; This creates an object empIdSeq of type Sequence in the database. To get rid of it, you must DROP SEQUENCE empIdSeq To use it: Insert into staff values (empIdSeq.nextval,’Harry’,’Foreman’); To see what the current value is; Select empIdSeq.currval from DUAL;

The full syntax for a sequence Create sequence sequencename Increment by n Maxvalue x / NoMaxValue Minvalue m / nominvalue Cycle / nocycle when max is reached, start at min again / not Cache c / nocache Oracle pre-generates and saves numbers in main memory. / not Order / noorder The numbers are generated in chronological order / not

Creating Indexes Sometimes it is necessary to look up data in an order that is not related to the key. To do this, an index can be used: E.g. CREATE INDEX ORDDATE ON SORDER (DELIVEREDDATE); This allows us to look up the orders by the date on which they were delivered. If I were in charge of taking in orders, this index may suit me very well. –Only use indexes when considered absolutely necessary. Every insert or delete must update any indexes that are declared for the table.