24 may 2011 in DevMix free course

1. T-SQL – 3rd session MedhatDawoud http://www.medhatdawoud.com MedhatDawoud@gmail.com Twitter  @Med7atDawoud

2. Agenda Nulls Order by Distinct Aggregates Grouping Having vs. where Compute by Union Constraints (PK,FK) Join tables Sub queries, nested queries Common restriction on Sub-queries

3. Nulls A null does notmean zero A null indicates that a value is missing, unavailable, incomplete, and inapplicable. Nulls represent an unknown quantity or value. Any question about a null could provide three answers: yes, no, or maybe Try it Now

4. Remember that studying null gives you the full ability to select any data from a database. The select clause specifies what columns we want to see The from clause tells what table we want to see data from The where clause restricts the data we will see

5. Order by The order by clause is used to specify a sorting order of the result set The sorting can be performed by column name or by column number. The default sort order is ascending (a-z), but you can specify a descending order by using the keyword desc. Try it Now

6. Distinct Try this: select studentName from libraryVisitors As you have seen from the above query result, you can get what appear to be duplicate rows in the result set From the scope of the result set, they are duplicates From the scope of the database they are not Sometimes we do not want to see these duplicate rows We can eliminate them by use of the distinct keyword Try it Now

7. Aggregates The three we will explore are count, sum, and average. countreturns a count of the number of rows in a table that match a certain criteria sumis used to add up all of the values in a column Avgwill return the average value in a column Try it Now

8. Group by Data in a table is essentially stored randomly The group by will order the data into groups You still need to specify an order by clause to perform sorting Nulls are consider a group The true power of a group by comes from using it in conjunction with an aggregate Try it Now

9. Group by Note that: One thing to remember is that if you use a group by with an aggregate, you must specify all non-aggregate columns in the group by clause. You can not specify an aggregate in the group by clause. Try it Now

10. Having vs. where There is a fundamental difference The whereclause defines the set of data the grouping is done on The havingdefines which groups are going to be returned to the user Having clause generally contain aggregates as part of the selection criteria The book "The Practical SQL Handbook" has a good explanation on pages 180 - 185 Try it Now

11. Compute by With a compute/computed by, you can only use columns in the select list A compute by is used to sub-summaries You can use any aggregate exceptcount(*) Compute by must start with the same expressions as listed after order by and not skip any expressions Try it Now

12. Compute by Legal order by a,b,c compute by a,b,c compute by a,b compute avg(price) by a Illegal order by a,b,c compute by b,a,c compute by c,a compute avg(price) by b Try it Now

13. Union combining data from two different tables when they have mutually exclusive criteria The only restrictions on unions are that the same number of columns must be in each separate result set and the datatypes must match We use the keyword Union to do that Try it Now

14. Relationships A database derives its usefulness from containing a group of tables that have some relationship to each other An entity is a person, place, or thing of importance to an organization An entity generally becomes a table Relationships are the connections between tables Relationships are usually implemented as keys in a database design

17. Primary Key A primary key is a special type of key that consists of one or more columns that uniquely identify a row Primary keys must be unique and can not contain null values A table will only have one primary key Primary key can not be null. Try it Now

18. Foreign Key A foreign key is one or more columns that refer to a primary key of another table If we have stu_id as a primary key in students table it’s consider to be a foreign key in the libraryVisitors table. Try it Now

19. Composite key A primary key and a foreign key can consist of more than one column When a key contains more than one column, it is known as a composite key The primary key of the titleauthor table is a composite (au_id,title_id)

20. Join tables All of the data in a database is segmented into tables and we generally need data from more than one table to show what we need You will notice that there is no such thing as a join clause in our SQL syntax If we get the data from library table we will get the stu_id which make no sense we need the studentName so, we use joins as shown in the try it out. Try it Now

21. Join tables (PUBS)

22. Join tables We are retrieving data from more than one table, so each table must be specified in the from clause So, The from clause can be seen as the main driver of a SQL statement The type of join we have examined so far is also referred to as an equi-join or an inner join And if you will notice that the results that has null values is ignored

23. Join tables To solve the problem of the null values we use another type of joins called outer join Outer joins come in three different flavors Left Right Full Left: stores.stor_id *= sales.stor_id Right: sales.stor_id =* stores.stor_id A full outer join is included here for completeness If you have one table with 100 rows and another with 1000 rows, a full outer join will produce a result set of 100,000 rows Try it Now

24. Full outer Join A full outer join will produce a cross product of the two tables This is because with a full outer join, you are telling the database to give every combination of rows possible The first time you inadvertently fire one of these off, you will get a rather angry call  from your DBA

25. Sub queries Sub-queriesare simply a SQL statement nested inside of another SQL statement. The most common place to do this is in a where or havingclause. Subqueries come in two basic kinds: correlated and noncorrelated

26. Two kinds of sub queries A noncorrelatedsub-query is one in which the inner query is independent, gets evaluated first, and passes it’s result set back to the outer query A correlatedsub-query is one in which the inner query is dependent upon the results from the outer query

27. Two examples to sub queries noncorrelated: select pub_name from publishers where pub_id in (select pub_id from titles where type = 'business') correlated: select pub_name from publishers p where 'business' in (select type from titles where oub_id = p.pub_id) Try it Now

28. Join and sub-queries select distinct pub_name from publishers, authors where publishers.city = authors.city AND select pub_name from publishers where city in (select city from authors) will return the same results Whether you use joins or subqueries is usually a matter of choice Most joins can be expressed as subqueries and vice versa

29. Common restriction Subqueries can not manipulate their results internally. i.e. They can not contain an order by or the keyword INTO You use the ANY and ALL keywords with a comparison operator in a sub-query > ALL means greater than every value in the results of the inner query (> maximum value) > ANY means greater than any value in the results of the inner query (> minimum value) Try it Now

30. Exist The last type of sub-query is used to test for the existence of something To find all of the publishers who publish business books we would do the following: select distinct pub_name from publishers where exists (select 1 from titles where pub_id = publishers.pub_id and type = 'business')

31. Nested Queries A sub-query may contain anothersub-query In fact you can nest as many levels as you need. However, for most applications more than four levels is an indication of poor database design

32. The End