Topics to be Covered

SQL Basics

Introduction

Before I begin, this article is intended to each and every beginner, whether a Software Developer or a Quality Analyst, this is a must know for every software engineer. Some might say we have Database Administrators, so why should we understand? I feel however that as developers we should have a thorough understanding of what is going on and be able to handle everything. This article will explain each and every concept in details to get started with the Structured Query Language (SQL). First of all, the question going on your mind now would be, what is SQL? As said previously, SQL stands for Structured Query Language. It is a special-purpose programming language. Its purpose is to manipulate relational databases. Relational databases are one that have detailed information about all the tables in a formally organized manner. The best part here is it contains both the data-definition syntax as well as data-manipulation syntax.

What a Database is

Now the diagram describes what it is! A database is simply a container that helps organize data in a format or constructive manner. In a database data is stored in a rational way. Now, let's take an example to help understand it better. For instance, our contact list in cell phones. If we think a bit into it we can understand that the contact list is a database with data in it in an organized manner. Then we sort by the letters to search the names from the list. Thus we perform a query execution on the list to retrieve the records we want. This is the flexibility a simple database provides. It makes it easier to: There are basically the following three types of databases available: We will be discussing only relational database since that is commonly used. I have provided the links to the other types of databases for reference.

What Relational is

The dictionary meaning of this is "Things that relate each other in a way". Here the meaning does keeps it consistency. The Relational Model is the database model on which SQL is based. It is thus a way to describe data and the relationship among those data entities. In a relational database, data is stored in a construct called a table. Now what is a table?

A small Example

Before starting with the rest, I hope the readers have a view of the SQL Server Management Studio (SSMS). Here is how the SSMS looks:



Now let's ask questions of ourselves. Why do we need databases with multiple tables at all, why not a single table? Let's see with an example why. Suppose we have a User table that would have columns named UserID, FName, LName, Email and PhNo. Now suppose we have only this table and there is a requirement in the future for a user to (optionally) have more than one Phone number or more than one Email. Just think how we would approch this, since we cannot store multiple values into a column. Thus, that does not provide us an indication to have another table!


Just see the difference below:


In the preceding figures, you can see the difference clearly, using the first table will cause a real problem and a lot of redundant data/records in the table that anyway violates the Normalization rules (Normalization Rules). Thus creating the secod table that would be having a foreign key relationship with the User table, can now store as many phone numbers as a user can have.

SQL Statement

Naming Things

This might be a bit controversial since there is nothing called the best way to do it. So we follow what is followed the most, or follow the conventions already used. This would differ from person to person. But the most important factor to keep in mind is to have consistency throughout. So that when we see the database the names of the tables define themselves. Some of the rules I am listing below:

Creating Things

There is an entire set of SQL commands that are required to create and modify the structures/records in the database.

CREATE DATABASE DemoDatabase; USE DemoDatabase; CREATE Table DemoDatabase.DemoTable(...);

Here I have only shown the structures, we will be discussing more of that ahead. Datatypes are very vital when we will be creating the tables in a database. Each column in a database has a restriction on the datatype of the data that can be stored in the table. Let's have a look at the datatypes.

Data Type Value/Space
CHARACTER Can hold "N" number of characters that are specified implicitly, to be exact statically that is the number of characters to be accepted in the vallue is set during column creation
VARYING CHARACTER Can hold "N" characters that is set dynamically, that is if MAX is set then it would accept any number of characters sent from the server into the column.
BINARY Stores Hexadecimal data, fixed length with a length of n bytes, where n varies from 1 through 8,000. Used for images and other sort of blobs.
VAR BINARY As the name suggests, it accepts value of varying length. When size is not specified, then default value is taken as 1.
SMALLINT Its length is 2 bytes and ranges from -2^15 (-32,768) to 2^15-1 (32,767)
INT Its length is 4 bytes and ranges from -2^31 (-2,147,483,648) to 2^31-1 (2,147,483,647)
BIGINT Its length is 8 bytes and ranges from -2^63 (-9,223,372,036,854,775,808) to 2^63-1 (9,223,372,036,854,775,807)
TINYINT Its length is 1 byte and ranges from 0-255. Can be used for values with Yes/No or the enumeration types that are limited to fewer values
BOOLEAN Stores TRUE FALSE values. When NOT NULLABLE, then default value is False, else it also accept NULL values.
DATE Stores only Date values, only YEAR MONTH and DAY values in the format YYYY--MM--DD
TIME Stores only time values, only HOUR MINUTE and SECONDS values of a day in the format HH--MM--SS
TIMESTAMP Stores both date and time. This is for when the date is combined with a time of a day with the seconds. The format is just concating both the date format and the time format

These are some of the datatypes, I have listed. The important thing here to keep in mind is, when creating the table, we never actually keep in mind. Thus the datatype plays a very important role if we consider the future. Suppose we use the number of users or the userid in a table datatype as a smallint or only an int even, then sometimes in the future maybe the number of users increases to a large extent and overflows the range of the datatypes. Then here the problem develops. You might have heard about the YouTube crash for the Gangnam song. The reason behind it was this the issue with the datatype for the number of views. It increased to such an extent that it crashed. The Relational Database Management System is based on the relational model. In simple terms, it is a type of database management that stores data in realated tables.

Everything about the SELECT statement

Introduction

As we have already discussed about the SSMS where we will be demonstrating our queries. After you read through this part of the article, you will be able to understand the details of using the SELECT statement, asking various types of questions to the select query. First of all, we store the data in a database to reuse it at some later point of time. Querying data is all about asking questions when retrieving data. Understanding the combination of English and the SQL helps us postulate the queries very easily. Let's create a database first. Look below at the query.

CREATE DATABASE QUERY
  1. CREATE DATABASE DemoDatabase;
The preceding query uses the master and creates the database named "DemoDatabase". You can also directly create:



Database using the SSMS UI. See the following images.



When in the first image as you see, the New Database is clicked, another window pops up as shown in the second image. Add the database name and there and it creates the database for you with a maxsize of unlimited. Then we create a Table for the storage of records.

CREATING A TABLE
  1. USE DemoDatabase
  2. CREATE TABLE [dbo].[User](
  3. [UserId] [int] IDENTITY(1,1) NOT NULL,
  4. [UserName] [nvarchar](50) NULL,
  5. [word] [nvarchar](50) NULL,
  6. [UserEmail] [nvarchar](50) NULL,
  7. CONSTRAINT [PK_User] PRIMARY KEY CLUSTERED
  8. (
  9. [UserId]
  10. )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
  11. ) ON [PRIMARY]

Now the preceding query creates a table for you, but we need to understand the commands used in the query since some seem understandable whereas some are not.

PAD_INDEX: Specifies the index padding. The default is OFF. More info.
STATISTICS_NORECOMPUTE: Creation and Recreation of index and auto update of the columns statistics. More info.
ALLOW_ROW_LOCKS: Enhances performances by locking the entire table rather than locking individual row. By default SQL sets to ON More info. ALLOW_PAGE_LOCKS: Set to TRUE/ON when there are chances of high concurrency to improve performance. By default SQL sets to ON More info.

Thus, we have created a database that now has a table named User. Now for your information, we can also create a table using the SSMS UI as we have done for the database. We open up the database tree structure and find Tables as a chile tree under it. We right-click on that and select New Table, we get a different UI in the query window now asking for the column names and the datatypes also to set the Nullable/NonNullable with a check box, then let's view the images: Add the Primary key column and set it as Primary Key as in the following:



Then we need to set the column as an identity column that will be auto-incremented by 1 everytime a new row is added. The developer need not bother about incrementing or setting the primary key. Set it as in the following:



Then we add all the columns with the datatypes mentioned previously and save the table with a proper table name. Now here the checkbox for the Nullable column plays a vital role. Think and specify the value here. Finally the UI looks as in the following:



Thus, now we have created a table also. Now it is time to add records/rows into it. We will be writing queries to insert the values into the table. Let's see how:

INSERT ROWS INTO TABLE
  1. INSERT INTO dbo.[User] (UserName,word,UserEmail) VALUES ('Suraj','abc123','[email protected]');
Here as you can see we have added only the three column values since the primary key userId is auto-incremented, in other words an Identity column. Thus, this adds the values. For a faster approach, usually developers use the SSMS UI here also to add the records. We right-click on the specific table into which we will be adding records, then click on the Edit Top 200 (default value) items. Then we see in a new query window the table columns with the rows of records already added and a default empty row where we will be adding the values as in the following:



Here we add the values in the row containing NULL values. The Primary key column is not editable since it will increment itself. The other columns are populated with the values to set the table records. Now let's return to our topic of discussion Select.

1. Select all records from the table
  1. SELECT * FROM dbo.[User]
This query is very simple and easy to understand. Here the FROM clause is used that is easy when we retrieve data from a single table. As said previously '*' relates to all the columns in a table. Thus, the preceding query returns all the records of all the columns. Another point here is if we select some columns out of all columns then we should/must qualify the column names in the Select list of the query. We can bring in the concept of "aliasing" in our next query that selects some columns in the table.

2. Select fewer columns from the table
  1. SELECT users.UserName as Name, users.word as PWD FROM dbo.[User] as users;
The preceding query returns only the User name column as Name and the word column as PWD in the result set. This is the power of aliases, it changes the column head names. The following is the sample result set:



Similarly you can use shorter Table alias names and use it throughout your query for convinence and select as many and the specific columns you need from the table. There are various ways for limiting the result sets we get. By default all the rows that match the From clause query are retrieved in the result set. There are two ways to limit the result set of the Select statement. One is to add conditions to the Query after the From clause. Another is to add the DISTINCT qualifier to the Select statement. Distinct is a result set qualifier. Also TOP (number) can be used to limit the result set. We will see the second type here and move to the condition type consequently.

3. Select Distinct Rows
  1. SELECT DISTINCT users.UserName as Name FROM dbo.[User] as users;
This query returns all the rows that are distinct based on the names values.

4. Select some rows from TOP
  1. SELECT TOP 500 users.UserName as Name FROM dbo.[User] as users;
This would return only the top 500 rows, no matter how many in total we get as the result set. Next we explain the conditions type.

WHERE Clause

This is used after the FROM clause and can be thought of as a search from the records using the conditions based on which we would be doing the result search. Searching with much more precision. The body of the where clause will be one or more expressions with return value as True/False. If the response is true based on the conditions for a specific row coming under the check, then the row is added to the result set that is retrieved finally from the query. A simple example here would be a get of a list of records/users whose name is "Suraj".
5. Select users with UserName as "Suraj"
  1. SELECT * FROM dbo.[User] as users WHERE users.UserName = 'Suraj' ;
This selects all the users with the username as Suraj, usually this is made as Firstname as Suraj. Thus the result set would look as in the following:



To master the WHERE clause in SQL we need to master how all the operators available behave. Let's have a look at the operators available with the WHERE clause in SQL and how they would behave:

Datatype Behaviour
Eqals '=' Returns true if both the conditions on left and right hand side matches
Not Equals to '<>' Returns true if both the conditions on left and right hand side do not match
Greater than '>' Returns true is the value on the left hand side is larger than the right hand side value
Less than '<' Returns true is the value on the left hand side is smaller than the right hand side value
Greater than or equal to '>=' Returns true is the value on the left hand side is larger than or same as the right hand side value
Less than or equal to '<=' Returns true is the value on the left hand side is smaller than or same as the right hand side value

Now let's have a look at the queries and how they behave when the operators are applied. Here I will be applying operators on the string values.

6. Select the users with UserName greater than "Suraj"

  1. SELECT * FROM dbo.[User] as users WHERE users.UserName > 'Suraj' ;
The result set here would return the name starting with letters greater than "S", but remember the check is for each letter until the greater than matches. Result set:


Thus the name starting with "T" is displayed in the result. Similarly less than and the other operators would work out.

7. Select the users with UserName less than "Suraj"
  1. SELECT * FROM dbo.[User] as users WHERE users.UserName < 'Suraj' ;
8. Select the users with UserName not equal to "Suraj"
  1. SELECT * FROM dbo.[User] as users
  2. WHERE users.UserName <> 'Suraj' ;



Then let's peep into the AND OR operators. As we all know each boolean expression can be combined with other boolean expressions. These are combined by using the AND OR operator. AND means that both of the boolean expressions are combined using AND, both must evaluate to True for the row to satisfy to be a part of the result set. Like for example a list of the users with a first name of Suraj and email as a specific value. Here as in the simple something is the English language and it is to be used here to evaluate and produce the result set.

9. Select the users with UserName "Suraj" and UserEmail "[email protected]"

  1. SELECT * FROM dbo.[User] as users WHERE users.UserName ='Suraj' AND users.UserEmail = '[email protected]';
The result set goes as in the following:



Thus here in the result set, we can check that both of the expressions on the sides of the AND operator turn out to be true for both of the rows in the result set. The OR keyword means that any of the expressions on the either side of OR operator can be true, then the row becomes a part of the result set. That is when any of the expression satisfies then the row is a part. Just keep in mind that if the first expression turns out to be true then the second expression is not evaluated. If the first expression turns to be false, then the second expression is evaluated to be checked.

10. Select the users with UserName as 'Suraj' or UserEmail as '[email protected]'
  1. SELECT * FROM dbo.[User] as users WHERE users.UserName ='Suraj' OR users.UserEmail = '[email protected]';
The result set is as in the following:



As we see first when the first expression, UserName as Suraj, is a part of the result set and also the rows with the UserEmail as "[email protected]". The BETWEEN operator acts on any column and taking two values checking the range. If a row value lies in the range specified in the BETWEEN condition, then the expression evaluates to True.

11. Select the users with age lying between specific values
  1. SELECT * FROM dbo.[User] as users WHERE users.Age BETWEEN 25 AND 50 ;
The result set is as in the following:



Thus the result set includes only those rows where the Age lies between 25 and 50, others are neglected. Between in inclusive of the values being checked. The LIKE operator is a special one that is used with strings. When we give a LIKE operator to the strings then it looks and search for those values of the string that matches the pattern specified in the LIKE condition. For LIKE '%' is used that is otherwise called wild card symbol. The % can be used anywhere in the string. Like for example, who are all the users with names starting with "S".

12. Select the users with the UserName starting with letter "S"
  1. SELECT * FROM dbo.[User] as users WHERE users.UserName LIKE 'S%' ;

This gives the result with all the username starting with S. Thus we write Like 'S%' wild card symbol specified after the letter means starting with. Specifying the wild card symbol at the beginning like '%S' means all usernames ending with S.
  1. SELECT * FROM dbo.[User] as users WHERE users.UserName LIKE '%C' ;
Thus this is the result set with the records where the column value UserName contains 'C'. IN operator is another special operator that requires column and a list of specified/valued values. Values can be of any datatype. If a row column value matches any of the specified values in the list used for the IN operator, then the row is added to the result set.
  1. SELECT * FROM dbo.[User] as users WHERE users.UserName IN ('Suraj' , 'test') ;
This gives the result set having all Users with user name Suraj and test. Thus the IN operator seems to overlap with the BETWEEN operator, but they are different in some scenarios. IS operator is less used and a special keyword that is usedto check for NULL values. An interesting thing to keep in mind is that the NULL values are not assignable to the '=' operator and its complement is not.

Conclusion

Thus here I have tried to specify the basics of SQL. As I have said, I have tried here something. Any queries, suggestions and concerns are most welcome. Please raise them and let's discuss and clarify them. Keep sharing.

References

MSDN Introduction to SQL by John Flanders. SQL authority by Pinal Dave W3 schools