SQL Server And Relational Database - Part Two

The server name is important because it is the server where you’re developing the database. If you’re working locally then by default here you’ll see your server name which is installed in your machine and if you’re working on the online server. Then you just need to provide the IP address of server name and then the authentication type will be SQL Server Authentication and provide its user name and password.

SQL Server is the database management system which has 2 parts.

  • Database Engine
  • Management Studio

It is the GUI which helps you to talk with Database Engine.

Sometimes when we’re working with SQL Server, then on SQL Server authentication we find some error on login.

SQL Server

Actually, it is because SQL Services are off. Just click on Windows Start button or Search “Services” in Windows 10 and Start SQL Server Service,

SQL Server

And now, you can easily login in to your SQL Server. Here we have a few options.

SQL Server

The server name is important because it is the server where you’re developing the database. If you’re working locally then by default here you’ll see your server name which is installed in your machine and if you’re working on an online server, then you just need to provide the IP address of server name and then the authentication type will be SQL Server Authentication and provide its username and password.

Now let’s suppose it depends upon your requirements. You’re developing the application and you have not deployed your application yet on any server then you can make one machine a remote server and you can connect the other laptops with this machine. And here you’ll use this machine server name username password. But you don’t know what is its username and password because we set these kinds of things  on SQL Server installation.

Just connect the machine with SQL Server from Windows authentication. And here you can set your new server username and password.

SQL Server

Security > Logins > sa > This popup will appear. Make sure you uncheck this checkbox. And then click on Status menu tab and make sure that your status is Grant and enabled.

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Now your username password has been set. Now you can log in through these credentials as well.

So, here we’re working with Chinook database. I’ve uploaded the file here. Just download the file and open it in SSMS and execute the queries by clicking on Execute button or press F5.

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And now, the database has been successfully created.

Database Diagram

Now, we want to see in the database diagrams how the entities are related to each other. Just right-click on Database Diagrams and add a new database diagram. Now, select all the tables for which you want to see how they’re related to each other.

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So, this is our Database Diagram. The vital part in designing any database is the ability to create relationships between your tables. Here we have an example of relationships in our database diagram. Every table has a primary key, the fields which you’re seeing in each table with the key is the primary key of that table. It is automatically generating the unique number for each item. One of the benefits of having that primary key is we can use it elsewhere, this primary key will become the foreign key in the next table.

There are really two kinds of relationships that we really care about.

One to Many

It is the most common. The relationship which you’re seeing in the above database diagram with this symbol.

SQL Server

Here, we create 2 tables where 1 record is related to multiple records in the second table.

Many to Many

It is not as common as one to many but it still quite common now. You often have to think about this one, because these business situations can often feel like one too many.

Let’s say we’ve author and title entity. One author might have written many books. This is one too many relationships.

SQL Server

So, here we add the foreign key field which has the value of the primary key of the author. One more important thing is, always keep in mind we make the foreign key attribute an independent table. Here, title entity is dependent upon the author entity. So, we create a foreign key in the title entity. So, here we add a new column for the title table.

SQL Server

But here is the issue, let’s suppose one book is written by many authors. Then what would we do here? Few people follow the rule of creating a foreign key attribute which I’ve mentioned above and create one more column field in the title entity. (Rule is: Always create the foreign key attribute in Dependent Table)

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However, adding new columns to your tables and particularly this kind of scenario where repeating groups or repeating columns is a really bad idea. And it is a very bad technique in database design.

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So we get rid off that idea. Some people think that I’m a little bit cheap and I do something quick and dirty and I put comma separated values in Author field with the help of JSON.

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So this is not the suggested approach as well. In fact, we’re going to solve this problem by getting rid of the author column entirely and we go back to the two completely detached tables and what we do to fix this is we add another table. This is the junction table.

The only reason for this table to exist is to join author title together. So, in fact, the name of the table would be author book.

In any relational database, we can’t express the many to many relationships directly. You can only do this with one to many relationships. This is how we’re expressing the entities with many to many relationships. In our Chinook database, we have an example of many to many relationships.

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  • One to One
    It is a not so common relationship. We can handle it by creating one more field in the same table or by creating a new table and referencing the first table with a second table with primary key and foreign key utility.
  • Recursive Relationship
    Let’s take an example of the employee. Let’s suppose in an organization different kinds of employees are working like Manager, Employees, CEO etc. All these objects are actually working in a company so they are all lying in the Employee table. One or More employees can have a boss, and the boss itself is an employee. This boss reports to the Manager of his department. This manager also an employee. So, this is how employees are inter-related to each other. That’s why here we’ve Recursive relation. We just create the foreign key of its own primary key. This is how we create the recursive relation of a table.

Note
When you design your database then always make the same name as your Foreign Keys in one table and the primary key in another table. It is the standardized nature to design the database.

Database Creation

Now, let’s suppose you want to create your own database.

Right Click on Databases > New Database > Name your Database

SQL Server

When we create a new database, here 2 files are also created along with the database.

  • DatabaseName_Log
    Every information relevant to the log of database like insertion, deletion, modification inserted into the log file.
  • DatabaseName
    It contains your actual data. This is your MDF file.

And here you can change the location of these files and if you want these files then you can get these files from this location.

Table Creation

As we create the database, similarly  inthe same way we create the table.

Just Right click on tables > Add New Table > And Define your table with attributes and datatypes. And if you want to make them required then uncheck the Null checkbox. And if you want to make it optional then check the Null checkbox. And when you save the definition of your table, it will automatically save with dbo schema. Dbo stands for the database object.

SQL Server

This dbo is the schema of your tables. SQL Server makes your tables and objects separate with the help of schema. Everything we create like tables, views, stored procedures lies into dbo by default. But if we want to separate out our tables or objects from this by default convention like you want to create the tables for HR, then you want to create the tables in HR schema, some tables we create for finance so we want dbo.

So, for this purpose, you need to create the schema first and when you’ll create the tables then we specify the schema of the tables. And so we can use different schemas for different tables.

Keep in mind, databases don’t understand the GUI. Everything we’re doing with GUIrelated to database, our every action is generating the SQL script behind the scene.

Data Types

Character string and Unicode character string: In the beginning, we have a system to represent the character, ANSI standardized. It just has 55 character options but if you want to write the small English alphabets, Urdu language, special symbols, counting numbers then we use Unicode. In Unicode, we store each character in 2 bytes. And in Non-Unicode, we just store only 1 character in 1 byte. Now after using Unicode, we have the option to use Chinese, Urdu, Persian any language we want to show.

Let me tell you 2 things when you’re studying the data types.

  • Precision
    A total number of digits is its precision. Like 2,147,483,648.12345 its precision is 15.
  • Scale
    And the scale of the above digit is 5. Total number comes after the fraction.

Here, we’ve some important links between data types documentation and tutorialspoint. Here, you’ll get an idea about data types very well.

Query Designer

One of the Nice Feature in SSMS, is the ability to do something called Query Builder to construct queries very quickly.

  • Open New Query Tab
  • Right Click on the Blank Query Tab and Select Design Query In Editor
  • Select the tables from where you need the data to show.
  • The Query will be constructed for you in the panel down in the form of Join because we are accessing the data from multiple tables.
  • Now Tick Mark the Required columns from the table windows to show into the new Select Statement.

SQL Server

Select Statement

Let me tell you one important thing. Selecting the different fields from the table is quite a common task but remember that data insertion, updating, and deletion are easy but to show the data is quite difficult. And we perform this kind of operation normally in our daily routine.

  1. SELECT * FROM Customer  

This is used to show all the fields of the table with data. Now, we want a few fields to show on the screen.

  1. SELECT CustomerId,  
  2.             FirstName,  
  3.             LastName,  
  4.             Email  
  5. FROM Customer  

Now, let’s suppose we want to show the fields with some meaningful heading like FirstName should display as Name or First name (with space) and Email with (username) so we use an alias here.

  1. SELECT CustomerId,  
  2.             FirstName AS ‘Name’,  
  3.             LastName,  
  4.             Email AS ‘User Email’  
  5. FROM Customer  

Insertion

Now, we want to insert the data into our table.

SQL Server

Look, this is our Customer table design. And if we provide the required fields, data will be successfully inserted into the table.

  1. INSERT INTO Customer (FirstName,  
  2.                              LastName,  
  3.                              Email)  
  4. VALUES ('Usama',  
  5.         'Shahid',  
  6.         'learnbeginners@gmail.com')  

As we’ve many other fields in a table, that’s why we’re specifying the field names to insert the values in it with the help of a pattern. And if you don’t provide the parameter names.

  1. INSERT INTO Customer  
  2. VALUES ('Usama',  
  3.         'Shahid',  
  4.         'learnbeginners@gmail.com')  

Then we’ll see the error because provided values are not matching with table definition. This is how we insert the data. Here we have the primary key is an identity as well which automatically becomes unique on a new entry. That’s why we’re not providing the CustomerId in the parameter because it automatically updates on a new entry in a table. But let’s suppose, you’ve set the field as primary key in a table and it is not identified and you want to uniquely identify the record then you have to provide your own values as well.

Let’s suppose Course entity has information related to courses. Now we want to place the unique courseID number in a table then obviously we’ll make this field as the primary key but we’ll not apply the identity on it.

Update

When we need to update our record then the Primary Id should be in our mind.

  1. UPDATE Customer  
  2. SET FirstName = 'Uwaish',  
  3.         Email = 'usama@gmail.com'  
  4. WHERE CustomerId = 60  

If we miss the WHERE Statement here, then it will lead to disaster. All the records in the table will be changed with this new information. So we need to be really careful when we need to update the data. Same the case with the Delete, so be careful about the WHERE clause.

Delete

  1. DELETE FROM Customer  
  2. WHERE CustomerId = 60 

Drop

If you want to drop your full table with data and schema then you drop the table with SQL query in this way,

  1. DROP TABLE table_name  

We can’t recover the DROP table.

Truncate

Basically, it deletes all the data from the table. Now you might be thinking that DELETE statement and Truncate statement both are doing the same task. But the only difference is we can use WHERE clause with DELETE but we can’t use WHERE with TRUNCATE.

We use TRUNCATE only when we want to refresh or reset the database as well on removing the address.

Important Note When You’re Working With Multiple Queries

When you’re working with SQL queries and you have a lot of queries in the same .sql file then always comment your update and delete statements. Because if you execute the complete SQL file by mistake or you might press F5 during working on databases then your all the records will modify/delete in the table. So the best practice is always commented your update and delete queries.

Just put (double dashes) before your query to comment your query.

- - Delete From Customer

Drop vs Truncate

The drop is used to remove existing entities. And Truncate statement is used to remove all the rows from a table without logging the individual row deletions. 

DDL vs DML

We can divide the database language into 2 different categories broadly DDL and DML.

DDL stands for Data Definition Language. It has a few different commands.

  • CREATE (for creation)
  • ALTER (for modification)
  • DROP (for deletion)

DML stands for Data Manipulation Language. Here, we manipulate or play our database.

  • SELECT
  • INSERT
  • UPDATE
  • DELETE
  • BULK INSERT (it imports a data file into database table or views in a user-specified format)

Sometimes when we’re working with a database, we have a lot of data in notepad or excel sheet. Then how do we insert it? Instead of writing queries again and again of Insert, we can import all the data with BULK INSERT utility of SQL.

MERGE

It performs UPDATE, INSERT, DELETE operations on the table in 1 transaction. So here we’ve 2 tables, source table and target table. And we’ll merge them. We’ll see merge statement with an example later on.

3 Pillars of Database

There are 3 major components of the database.

  • Database Designing
    Here we’ll deal with different tables, and how many tables we’ll use in our system. What are their attributes? How we create the relationship among tables? This kind of things deals with database designing.
  • Database Development
    Here we develop our database with SSMS GUI or with Query.
  • Database Administration
    Suppose Database is increasingly enlarging in size. And let’s suppose the machine where you’ve installed your database server if it is crashed then what we’ll do. So here we take the backup and restore it and make sure the fine tuning of the database.

So designing is the most important part where you learn the different concepts of a database. If we know the database anomalies and how they come in the database then obviously we make the solution of this kind of problems.

Database Anomalies

When we talk about the database, we meant to say about data storage and data retrieval from the database.

So let’s suppose you’re writing something on the notebook and you’re writing something again and again. This is what we say data redundancy. And let’s suppose we’re talking about one person and we put some data of him like NIC, now somewhere else I again put his NIC but different from the previous one. This is what we say data inconsistency. Now might be 1st NIC is correct or the 2nd one is correct or both are incorrect. This kind of problems we tackle in the database.

These kind of logical mistakes are database anomalies which we handle with the help of Normalization. We’ll explore Normalization in later articles.

First, we explore the problems which can occur in the database.

Roll No. Name Class NIC City Course
1 Zeeshan 9th 12345 Burewala Math
1 Zeeshan 9th 12345 Burewala Islamiyat
1 Zeeshan 9th 12345 Burewala Physics
2 Waqar 10th   Arifwala Arts
2 Waqar 10th   Arifwala Chemistry
2 Waqar 10th   Arifwala Science

Insert Anomalies

Suppose 1 student is enrolled in a subject and if he enrolled again and again in different subjects then obviously we replicate the data of student again and again this is redundancy.

Now let’s suppose our institute launches one more subject like Social Studies. But there is no student is enrolled in this subject yet. Then we insert the information of the subject in this table.

          Social Studies

We just add the name of the subject -- it doesn’t enter the name of the student. Right? It means remaining column contains null values. This is why we say insert anomaly.

Delete Anomaly

Now let’s suppose Waqar has left the institute and now he’s gone somewhere else. Now he’s not here. So if I delete the Waqar records from the table then obviously the subject information will also be deleted. This deletes anomaly.

It means when we want to delete some kind of information then it also deletes some kind of other information as well. This deletes anomaly.

Update Anomaly

Here we’ve Zeeshan is enrolled in 3 subjects. Now we want to change the name of Zeeshan, so on the foundation of the name of Zeeshan in the table. If I update 2 records and left the 3rd one by mistake, then we’ve 2 places on the table where the data is correct and the 3rd one is actually our mistake. This is what we say data inconsistency.

And let’s suppose we want to update the name of the subject from Math to Mathematics, and if we update the record in one place and change it with mathematics and leave the 2nd place with just the math name this is why we say data inconsistency. Now we can’t realize the correct entry.

Our Dream

So our dream is we want to do something like if we apply the CRUD operations on the database then one thing should not affect the other one. It should be redundant free and data should be consistent. This kind of thing we handle with Normalization.

This is why normalization is important. Most of the time developers just working on the things and try to learn them but even they don’t know why they are doing that. So the precious recommendation is first of all try to find the problem, actually why we’re doing that why we need it. Then if we come on to the solution of this problem we can learn more.

Database Keys

Every entity has its own unique attribute like Book has unique ISBN number, every person has a different name, every electronic device has its own Mac Address, every phone number has its own unique identification number. So these are the candidate keys through we can identify our entity uniquely.

Different fields or attributes which we can use to identify that entity record as unique is/are candidate keys. Again in simple words, we see in the table how many columns exist in the table which can identify the unique record  independently. So one table can have many candidate keys like roll number, NIC, mobile number etc. And if we combine two or more candidate keys to make the record unique, is what we say composite key. The unique key is the column where the values can’t repeat like a student has its own NIC number which can’t assign to anyone else. So we can say that this column value is unique but might be it can have a null value in this column. So this constraint is unique but nullable.

Now, let’s talk about the primary key, let’s suppose we’ve 4 or 5 candidate keys in a table. So we make a strong field as primary key which can’t be null and which can identify the record uniquely in every condition. Now we use this primary key in multiple tables which becomes our foreign key.

Normalization

We’ve already seen that there are anomalies in the database and we resolve them with normalization. Suppose you’ve existing database or you’re creating a new database. And you know that if you add something here then you might get a logical redundancy in the table or some type of inconsistency occurs. So you design your database by taking care of such things.

Roll No. Name Class NIC City Courses
1 Zeeshan 9th 12345 Burewala Islamiyat, Math, Physics

1st Normal Form

Each cell should have unit value.

Any field doesn’t contain more than 1 value. So it becomes,

Roll No. Name Class NIC City Course
1 Zeeshan 9th 12345 Burewala Math
1 Zeeshan 9th 12345 Burewala Islamiyat
1 Zeeshan 9th 12345 Burewala Physics

Now it is in the first normal form.

Second Normal Form

And most of the time we see tables which are already in the first normal form. So before talking about the second normal form, let’s understand the concept of Functional Dependency.

Functional Dependency is totally the concept that mathematics come from functions. And dependency means if our values are dependent upon other values or not. This means that where the roll number 1 comes, the 9th class also comes with roll number records. Same as it is when we put roll number 1, CNIC also dependent upon this specific student. All these fields in the table are dependent upon the roll number one. So we can say that all these columns are functionally dependent upon roll number one. But as we can say Course information varies in each record of this roll number one. It means Courses are not dependent upon roll number.

So, we need to identify the attributes which are not fully functional dependent upon the primary key. And we make them separate in two different tables. Now, just think about the relationship between student and course entities. Obviously, one student can enroll in multiple courses and in one course many students can enroll. So the relationship is many to many between two entities. And we know that when we’re dealing with many to many, we need to make an extra third table.

Roll No. Name Class NIC City
1 Zeeshan 9th 12345 Burewala
2 Waqas 9th 12346 Arifwala

Student

Serial Number Course Name
1 Physics
2 Chemistry
3 Math

Course

And now the third table comes into the picture,

Roll Number Serial Number
1 1
1 3
2 2

StudentCourses

Now the entities aren’t dependent upon each other. Let’s suppose if we need to add one more course in the table then there will be no effect on Student entity. And if we add one more student to the table and if he doesn’t enroll in any course then there will be no impact on the Course entity.

So the second normal form in dependent column should rely on the primary key of its record.

Important Point

If you move back to the first normal form and if we want to extract the data from the table then it really it would be very easy and fast to extract the data from a single table but there will be a redundancy in the table. But after applying the second normal form, our 1 table has broken into three parts. Now data retrieval is not so easy, it takes time because now we can’t extract the data directly. Now there is a relationship among tables.

Let’s suppose you want to know the courses enrolled by Zeeshan, then you move to the Student table and pick the Student_id and then come on to the mapping table and extract the Serial Numbers of Courses and then move to Course table. This is how it works. That’s why we use joins here for data retrieval after applying normalization on the tables. This is the disadvantage of Normalization. That’s why sometimes we keep our data denormalize. We’ll discuss it later on.

Third Normal Form

Now let’s suppose, here is a table

Roll No. Name Class Birth City Province Country
1 Waqas 9th Arifwala Punjab Pakistan
2 Zeeshan 9th Lahore Punjab Pakistan

Now if you focus on this table, Province and Country are dependent upon the non-primary key attribute (Birth City) because Province and Country are dependent upon the Birth City. This is what we say Transitive Dependency among attributes. Here similar information is repeating again and again. So we again separate this information from the base table.

Now, one student belongs to only one city. So it is one to one relationship.

Roll No. Name Class BirthCityId
1 Waqas 9th 1
2 Zeeshan 9th 2

Student

And now our City Table is,

City_Id Birth City Province Country
1 Arifwala Punjab Pakistan
2 Lahore Punjab Pakistan

Note
When you design your database then always make the same name for your foreign keys in one table and the primary key in another table. It is the standardized nature to design the database.