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Advanced SQL Tutorial

This advanced SQL tutorial focuses on the design of more complex SQL statements and the strategies available for implementing them, it concentrates on sub queries and joins because they are often inter- changeable, and views because these are useful for hiding the complexity of queries involving sub-queries and joins. As this is an advanced tutorial there is some consideration of performance issues, but this aspect is more thoroughly explored in our series on Oracle performance tuning. You might also be interested in our beginner's SQL tutorial, our PL/SQL tutorials or our other Oracle tutorials. Part 1 of this advanced SQL tutorial (this tutorial) introduces sub-queries and looks at non-correlated sub- queries. Part 2 looks at the use of sub-queries. Part 3 looks at nested sub queries, sub-queries in the from clause and sub-queries that return no rows. Part 4 of this tutorial covers correlated sub-queries.

Sub Queries

There are two types of sub query: correlated and non- correlated. The difference between them is that a correlated sub query refers to a column from a table in the parent query, whereas a non-correlated sub query doesn't. This means that a non-correlated sub query is executed just once for the whole SQL statement, whereas correlated sub queries are executed once per row in the parent query. In this tutorial we’ll look at non-correlated sub queries. See advanced sql tutorial part 4 to learn about correlated sub queries. Sub queries are also known as nested queries) and are used to answer multi-part questions. They are often interchangeable with joins and the Oracle optimiser may well treat a sub-query exactly as if it were a join. Let’s take the trivial example of finding the names of everybody who works in the same department as an employee called "Jones" to illustrate this. We can write the SQL for this in 2 ways - as a sub query: SELECT name FROM emp WHERE dept_no = (SELECT dept_no FROM emp WHERE name = 'JONES') or as a join by joining the emp table to itself:- SELECT FROM    emp e1               ,emp e2 WHERE e1.dept_no = e2.dept_no AND      e2,name = 'JONES' With the first example we have to be careful to ensure that the subquery return no more than one row, otherwise Oracle will generate an exception at run time. To be safe we can  change "=" to "IN" as shown in this next example. SELECT name FROM emp WHERE dept_no IN (SELECT dept_no FROM emp WHERE name = 'JONES') for such a simple query like this there would be very little difference in terms of performance of the SQL as a subquery or as a join, but with more complex queries there could well be performance implications. For this reason it is always worth trying a few variations of the SQL and examining the execution plans before deciding on a particular approach, unless they're very simple queries. Learn more about Oracle performance tuning here. Continue this Advanced SQL tutorial and learn more about non-correlated sub-queries or see part 4 to learn about correlated sub queries.  --------------------------------------- Looking to sky-rocket productivity, save time and reduce costs? Training is a highly cost-effective and proven method of boosting productivity. Smartsoft offers instructor-led training in Oracle and related technologies on or off site in cities across the UK as well as self-study online training. See our scheduled Oracle training courses, or let us know your requirements. Oracle tips and tricks Subscribe to our newsletter, jam-packed full of tips and tricks to help you slash costs, sky-rocket productivity and make your systems better, faster and smarter. Smartsoft Computing Ltd, Bristol, England Tel: 0845 003 1320 Need help with Oracle? Contact Us View our privacy policy This site uses to gather statistical information about our visitors. This data is aggregated to show industry trends (such as browser share). However, this data shall be the average of many thousands of visits and is in no way linked to individuals. View woopra privacy policy.  Oracle and Java are registered trademarks of Oracle and/or its affiliates. Other names may be trademarks of their respective owners. © Copyright Smartsoft Computing Ltd 2014. All rights reserved.
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