SQL in Simple English - Part 1

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What is SQL?

SQL stands for "Structured Query Language".

What does that mean?

There are a lot of Databases available in the market such as MS Access, Oracle and many others. For you to write programs that interact with these databases easily, there has to be a way where you could get information from all these databases using the same method. For this purpose SQL was developed. It is a kind of language (simple when compared to the likes of C or C++) which enables you to ask all your queries to a database without bothering about the exact type of database.

What the heck is SQL?

Ok lets get straight to the point. Suppose you have a database which has a table called people. (I hope you know what tables are). And you want the details of all persons whose firstname is 'Reena'. So you could use a SQL statement as follows

SELECT * FROM people WHERE firstname = 'Reena'

When you use this Query the database engine would first find the table called people. Then it would find a column called firstname. Next it would compare all the values in that column with 'Reena'. Finally it would return all the details wherever it finds a match for the firstname.

Tell me something more of the bigger picture.

When you write a database program in VC++ or Java or any other language for that matter, you would make a database connection to your database and then you would query the database using SQL queries. When you query the database with any SQL query the database returns a recordset. A recordset is basically a set of records (all the entries that your query returns). This recordset is received in your program and all languages have a data structure which represents a recordset. Once this data structure (in your program) gets populated with the results from the database query, your could use a for loop to loop through all the entries.

How do I connect to a Database through my program?

Hey guys.. this is a tutorial on SQL.. so I wouldn't be focusing on those aspects in this series


What was that thing about recordsets?

When you connect to a database and execute SQL Queries, the results of the Query are returned back to your program. This returned data has to be stored in some Object or Data Structure within your program to be used by your program. Once you store the results in this Object you no longer have to be connected to the Database. For a more detailed explanation please refer to a book on Database programming.

What are you going to discuss now?

Now you will learn basic SQL statements such as SELECT, INSERT, UPDATE and DELETE.
For all the examples in this article we would be using a sample database table which is shown below

Table Name : people

lastname     firstname     age     address     city
pai     Kiran     22     Mahavir Nagar     Mumbai
Hunter     Jason     41     Oak Street     San jose
Kanetkar     Yashwant     38     RajaBhai Street     Nagpur

What is SELECT statement? How do I use it?
The SELECT statement lets you select a set of values from a table in a database. The values selected from the database table would depend on the various conditions that are specified in the SQL query. Here are some ways to use the SELECT statement. I have listed the SQL statements and the respective results that you would obtain if you would execute those queries.

SELECT lastname,firstname FROM people
Would return a recordset with 3 records. Each record would have 2 values.
The first record would have 2 values 'Pai' and 'Kiran'. Whereas the second record would have 2 values 'Hunter' and 'Jason'.

SELECT * FROM people WHERE firstname='Jason'
Would return a recordset with 1 record. The record would have 5 values in that, namely 'Hunter' , 'Jason' , '41' , 'Oak Street' and 'San Jose'.

SELECT * FROM people WHERE age>25
Would return a recordset with 2 records.

Note : That whenever you are comparing a varchar the value should be enclosed in single inverted commas ( ' ). Whereas when you compare an integer the value need not be enclosed in single inverted commas.

How can I compare a part of the name rather than the entire name?

SELECT * FROM people WHERE firstname LIKE '%an%'
Would return a recordset with 2 records. This statement would return 2 records since the sequence 'an' occurs in 2 firstname values, 'Kiran' and 'Yashwant'.


Can I use Boolean operators such as AND or OR to make complex queries?
Good news!! Yes you can.. Actually as a matter of fact, once you start developing professional database applications you would almost always use such Boolean operators to make effective queries.

SELECT address FROM people WHERE (firstname='Kiran' OR city='Nagpur') AND age>30
Would return a recordset consisting of 1 record with 1 value only. Since AND condition specifies that the firstname of the person could be 'Kiran' or his city could be 'Nagpur' , BUT that person has to be over the age of 30. The recordset would have only 1 value in it : 'Rajabhai Street'.

 

What is the INSERT statement? How do I use it?
The INSERT statement lets you insert information into a database. A few examples are shown below

INSERT INTO people VALUES ('Bush', 'George', 47 , 'White House', 'Washington')

Would insert a new row at the bottom of the table people consisting of the values in parentheses in the above statement.

INSERT INTO people (lastname, city) VALUES ('Gates', 'Redmond')
Would insert a new row at the bottom of the table people consisting of only 2 values as present in the above statement, namely 'Gates' and 'Redmond'. The remaining columns for that particular record would be left empty (null).

Note : A null value is different from 0 or ''(Empty String). A perfect example of this would be a column describing the hair colour for many people. In case the person is bald then the value of the colour should be null rather than empty. This would be perfect from the database design view. A particular entity which doesn't exist should be represented similarly and not by empty Strings.

How do I delete a record from a database?
Use the DELETE statement to remove records or any particular column values from a database.

DELETE FROM people WHERE lastname = 'Hunter'
Would remove the entire record which represents any person whose lastname is 'Hunter'. In our case it would remove 1 record from the sample database table people. It would remove all the values that were a part of that record.

Is there a way to update any record in a database?
Yes. You could use the UPDATE statement. The update statement updates (or replaces) those values that were specified in the SQL statement with the new values provided.

UPDATE people SET age = 50, city = 'Mumbai' WHERE (lastname = 'Hunter' AND firstname='Jason')

Would change Jason Hunter's age from 41 to 50 and would make him shift his residence from 'San Jose' to 'Mumbai'. Isn't that cool?? A new Java Guru is Mumbai !!


Are there better ways to use SELECT?
Yes there are.. and now you will learn some of the better ways of using the SELECT along with some other SQL terms such as DISTINCT , ORDER , MAX , MIN , AVG , etc..

For all the examples in this article we would be using a sample database table which is shown below


Table Name : people

lastname     firstname     age     address     city
Gates     Anthony     11     Circuit city     Bangalore
Hunter     Jason     41     Oak Street     San jose
Gates     Bill     59     Microsoft Corp     Redmond
Kanetkar     Yashwant     38     Rajabhai Street     Nagpur

How could I get distinct entries from a table?
The SELECT statement in conjunction with DISTINCT lets you select a set of distinct values from a table in a database. The values selected from the database table would ofcourse depend on the various conditions that are specified in the SQL query. Here are some ways to use the DISTINCT keyword.

SELECT DISTINCT lastname FROM people
Would return a recordset with 3 records. Each record would have 1 value in it. So basically the first record would contain 'Gates', the second would contain 'Hunter' and the third would contain 'Kanetkar'. Inspite of the lastname 'Gates' being present twice in the table, only one occurrence of it will be considered since the DISTINCT keyword was used in the SQL statement.


Is there a way to get the results of a Query sorted in any order?
Yes there are ways which will sort the results and return the sorted results to your program.. thus saving you the pain of carrying out the sorting yourself. The ORDER BY keyword is used for sorting.

SELECT firstname, age, city FROM people ORDER BY firstname
Would return a recordset with 4 records. Each record would have 3 values corresponding to firstname, age and city. But the specialty of this statement is that the records would be sorted according to the firstname in ascending alphabetical order (A first - Z last).
e.g. The first record would be that corresponding to the person whose firstname is 'Anthony' , followed by 'Bill' and then 'Jason' and finally 'Yashwant'.

SELECT firstname, age, city FROM people ORDER BY firstname DESC
Would return a recordset with 4 record as the above case, but this time the records would be in the reverse order. Namely the first record would be 'Yashwant' and the fourth one would be 'Anthony'
How can I find the total number of records in a table?
You could use the COUNT keyword in many ways.. here are some ways.

SELECT COUNT(*) FROM people WHERE age>40
Would return a recordset consisting of 1 value. Contrary to previous SQL statements the COUNT statement return one value which directly indicates the total number of records existing in the database that fulfill your conditions
e.g. In our case the above statement would return a value of 2

SELECT COUNT(city) FROM people
Would return a recordset consisting of 1 value. And that value would be equal to 4. The important point to note is that this statement return the total number of Non-Null entries only.

SELECT DISTINCT COUNT(lastname) FROM people
Would return a recordset consisting of 1 value. And that value would be equal to 3. Remember that when you use the COUNT keyword you do not get the actual lastname of the persons but you only get the total number of records that exist in the database that match your requirements. And in this case since DISTINCT was also used it would find the total number of records where there are distinct firstname only.

I heard there is some mathematical stuff in SQL?
Yeah.. there are many simple operations that you could do in order to formulate some useful information from a database rather than getting simple records from the database. Here are a few examples of these mathematical operations

SELECT AVG(age) FROM people
Would return 1 value corresponding to the average age of all the persons that exist in the table people.

SELECT AVG(age) FROM people WHERE age>30
You should be able to figure that out yourself.. if not please start reading right from the first article in this series ;-)

SELECT MAX(age) FROM people

Returns the maximum age among all the persons in the table people.

SELECT MIN(age) FROM people
Returns the minimum age among all the persons in the table people.

SELECT SUM(age) FROM people WHERE age>20
Returns the total sum of all the ages of the persons whose age is above 20 from the table people.

How do I delete a record from a database?
Use the DELETE statement to remove records or any particular column values from a database.

DELETE FROM people WHERE lastname = 'Hunter'
Would remove the entire record which represents any person whose lastname is 'Hunter'. In our case it would remove 1 record from the sample database table people. It would remove all the values that were a part of that record.


Is there a way to update any record in a database?
Yes. You could use the UPDATE statement. The update statement updates (or replaces) those values that were specified in the SQL statement with the new values provided.

PDATE people SET age = 50, city = 'Mumbai' WHERE (lastname = 'Hunter' AND firstname='Jason')
Would change Jason Hunter's age from 41 to 50 and would make him shift his residence from 'San Jose' to 'Mumbai'. Isn't that cool?? A new Java Guru is Mumbai !!


Show me something new in SQL..
Ok here are 2 new things that I have used only a few times in my programs. But they maybe useful to you,so I shall talk about them. There are 2 keywords called GROUP BY and HAVING.

Both these are used in conjunction with the aggregate statements like SUM , AVG , etc..

For all the examples in this article we would be using a sample database table which is shown below
Table Name : companies name

name     profit
Sega     25000
Microsoft     50000
Sega     10000

So what is GROUP BY? When do I use it?
The GROUP BY keywords have been added to SQL because aggregate functions (like SUM) return the aggregate of all column values every time they are called. Without the GROUP BY functionality, finding the sum for each individual group of column values was not possible.

SELECT name, SUM(profit) FROM companies
Returns a recordset with 3 records. Each record has 2 values. The first record would have the value 'Sega' and '85000'. The second record would have the values ' Microsoft' and '85000'. And the third record would have the values 'Sega' and '85000'.
Thus it is clear that this is not what was required. There is no sense in getting the sum of all the profits of all the companies along with each company name. What would be acceptable is the sum of all the profits of the respective companies along with that company's name. Read the next statement..

SELECT name, SUM(profit) FROM companies GROUP BY name
Returns a recordset with 2 records. Each record has 2 values. The first record would have the value 'Sega' and '35000'. The second record would have the values 'Microsoft' and '50000'.

And what is the HAVING keyword?
The HAVING keyword has been added to SQL because a WHERE keyword can not be used against aggregate functions (like SUM). Without the HAVING keyword it would not be possible to test for function result conditions.

SELECT name, SUM(profit) FROM companies GROUP BY name HAVING SUM(profit)>40000
Returns a recordset with 1 record. This record would have 2 values, namely 'Microsoft' and '50000'. Since the sum of the profits of the company by the name 'Sega' is only 35000 (which is lesser than 40000 as required in the Query)

SELECT Company "Firm", Amount "Profit" FROM Sales
Alias - Returns the 2 cols with the heading as "Firm" and "Profit" instead of "Company" & "Amount"
Do I have to work with the same column names that exist in the database tables, within my program?
Yes and No. As far as the Query is concerned, you have to include the column names that exist in the Database table in your Query. But you can also include an alias with which you can carry on further work with the returned results by the Database. Let me show you an example of using an alias

SELECT name "firm", profit "riches" FROM companies
Would return a recordset consisting of 3 records each with 2 values. Basically all the 3 records from the sample database would be returned , but the column names would be changed to those that were mentioned in the SQL statement. Thus name would be changed to firm. And profit would change to riches.