how to work with Multiple tables in SQL for tester– Understanding joins

We have gained knowledge on basic queries in SQL working with a single table in previous articles. Let us extend our knowledge working with multiple tables in SQL. Below are the tables, we will use in our examples.

This post will mainly discuss on concept of joins in SQL.In the next post, we will go further and discuss on nested queries, union, and other useful concepts on SQL.

Problem 1: From the above tables, provide Org_Name for each of the users in User Table?

Answer: First of all, we need to identify which information we need to extract from above table. We need Org_Name from Organisation table and Username from User Table.

Second Point is to identify the common attribute/column in two tables based on which we will create the relation between two tables. As in above tables, Org_Id is the foreign Key in table User which is mapped with Id in the Organisation table and is used to create a relation between the two tables.

We can use joins to get data the two tables. JOIN is used to combine rows from two or more tables, based on a common field between them. In this problem, we will use join or inner join or a equi – join to extract the required information

•  INNER JOIN: Returns all rows when there is at least one match in BOTH tables. An Inner Join is same as Join or equi-join.

•  Since we have make a match between Organisation and user table on Org_Id, we will create inner join or equi-join or simply join as shown in example below

• Syntax - Join : Select * from Organisation a join User b on a.Id = b.Org_Id

• Syntax - InnerJoin : Select * from Organisation a innerjoin User b on a.Id = b.Org_Id

• Syntax – Equi-Join : Select * from Organisation a,User b where a.Id = b.Org_Id

Join and Inner Join are same thing and fetches the same results as Equi-Join between two tables.

Other than Inner Join or equi-Join, Other examples of joins are following:

• LEFT JOIN: Return all rows from the left table, and the matched rows from the right table

• Syntax- Left Join : Select * from Organisation a LEFT join User b on a.Id = b.Org_Id

• RIGHT JOIN: Return all rows from the right table, and the matched rows from the left table

• Syntax- Right Join : Select * from Organisation a Right join User b on a.Id = b.Org_Id

• FULL JOIN: Return combined result of both LEFT and RIGHT joins.

• Syntax- Full Join : Select * from Organisation a full outer join User b on a.Id = b.Org_Id

In above problem, we will use an inner join and can use any syntax from Join, inner-join or equi-join.Since we require Org_Name and User_Name in Output, Instead of fetching all information using Select *, we will extract Org_Name from Organisation table and similarly User_Name from User table. Also we can use alias to giving meaningful name to column. The syntax will be :

Select a.Org_Name as OrganisationName ,b.User_Name as User from Organisation a join User b on a.Id = b.Org_Id

Problem 2: In the above tables, provide Org_Name, Salary and Designation for each of the user in User Table?

Answer: In this example we need to create multiple join between tables. The query will be as follows. Try to understand the query with the concept explained in previous table.

Select us.User_Name, org.Org_Name, us.Salary, des.staff_role from User us join Organisation org on us.Org_Id =org.Id join User_Designation usd on us.Id = usd.User_Id join Designation des on usd.Designation_Id = des.Id

Basic of SQL a tester must know – II

In the first post on SQL, We discuss on very basic queries in SQL , In this post we will discuss further from where we left in post - 1

The post is prepared using SQL Server, and there might be slight change in the query in Oracle or other database servers. We will cover these wherever applicable. Let us continue with table name Organisation with following details as shown below:

Let us discuss on more queries from where we left in the previous article.

•   Query to get top records from the table

•    Select Top 2 from Organisation

•    Syntax: Select top number|Percentage from [tableName].

•  The above query is specific to SQL Server, The syntax for the same in MySQL and Oracle is as follows:

•    Select * from Organisation where rownum<=2   --- Oracle

•    Select * from Organisation limit 2   --- MySQL 

• 
  

• Query to search for multiple values in a column in SQL – Using In Operator or between Operator, we can search for multiple values for a specific column

•   Select * from Organisation where Org_Name in (testorg1,testOrg3)

•  Syntax: Select * from [tableName] where columnName in (value1,value2) 

•  Query to search for multiple values in a Column in SQL – Using between Operator. The below example will list all Organisation with values of Org_Code between 10 and 2000.

•   Select * from Organisation where Org_Code between 10 and 2000

• Syntax: Select * from [tableName] where columnName between  value1 and value2

• Query to search for multiple values in a Column in SQL – Similar to Between , We can use not Between Operator. The below example will list all Organisation with values of Org_Code excluding values between 10 and 2000.

• Select * from Organisation where Org_Code not between 10 and 2000
•  Syntax: Select * from [tableName] where column_Name  not between  value1 and value2

•  Query to search for multiple values in a Column in SQL – Similar to In and Between, We can use LIKE operator together with Wild cards to search for a specified pattern in a column.

•   Select * from Organisation where Org_Name like ‘%test%’.
• In the example % acts as wild card to search for pattern which test displayed anywhere in the Org_Name. Other examples include:
• Select * from Organisation where Org_Name  like '_ estOrg2'; (wild card for one character)
• Select * from Organisation where Org_Name  like '[tfd]%’; (wildcard for all org_name starting with t, f, or d)

• Using Group By with aggregate functions can help us to extract useful information from the tables. For e.g In above Organisation table, we need to know how many organizations’ belongs to Organisation Type ‘Auto’ and how many to ‘Manual’. We can use group by Clause together with aggregate function as shown below:

•     Select Org_Type , count(*) as CntOrg from Organisation group by Org_Type.
•       The best practice is to provide Column names used in Group By Clause in Select Clause also to generate meaningful information.
•       We can use multiple columns in Group by Clause, e.g : Select Org_Type, Is_Active, count(*) from Organisation where Org_Type is not null group by Org_Type,Is_Active.

• We can use having clause to search for required in SQL for aggregate function similar to Where clause for a column in table: 

• e.g : Select Org_Type, count(1) as Org_Cnt from Organisation group by Org_Type having Org_Cnt>2.
• 
  

•   Different Aggregate function used in SQL  are as follows:

• Sum() - returns the total sum of a numeric column.

• Avg() - returns the average of a numeric column.

• Count() - return the count of rows satisfying condition

• Max() - returns them ax value of numeric column

• Min() - returns the minimum value of a numeric column

• The syntax for aggregate function is :  Select aggregate_Function(column_Name) from Table Name  e.g: Select max(Org_Code) from Organisation.

In the first two posts in SQL, we are done with the basic concept of SQL useful if we need to extract information from a single table. In the next post, we will discuss on string functions, joins and working with multiple tables.

Basic Queries in SQL a tester must know - I

Making SQL query of database is must for a tester to know. In this blog, I will explain some basic query of SQL must for a tester to know. In later posts in this series, we will discuss more on nested queries, joins,working with conditional loops in sql and other useful concepts from a tester point of view. The post is prepared using SQL Server, and there might be slight change in the query in Oracle or other database servers.

To start with, let us assume, there is a table name Organisation with following details as shown below:

Let us know some basic queries to fetch information from a single table.

•     Query to get all information from the table
•    Select * from Organisation  
•   Syntax: Select * from [tableName]

•     Query to get particular column(s) information from table
•  Select Org_Name, Org_Code from Organisation
• Syntax : Select [Col_Name1], [Col_Name2] from [tableName]

• ·  Using Where Clause to get subset of information
•    Select * from Organisation where Org_Type = ‘Auto’
• Syntax: Select * from [tableName] where [Col_Name1] = <expected value>                                                      
•    Using Where Clause to get subset based on multiple condition in where clause
•      Select * from Organisation where Org_Type = ‘Auto’ Or Org_Code = 201
• Syntax: Select * from [tableName] where [Col_Name1] = <expected value> Or [Col_Name2] = <expected value> 

•    Using alias to get more readable and presentable form
•     Select Org_Name as Organisation Name from Organisation
• Syntax: Select [Column Name] as AliasName from [TableName]
•    Using Distinct to get distinct records of a table
•    Select distinct Org_Type from Organisation
• Syntax: Select distinct [Column Name] from [TableName]
•     To get multiple distinct combination use:
•   Select distinct Org_Type,Org_Code from Organisation
• Syntax: Select distinct [Column Name] from [TableName] 

•     Using Order by to sort the data in their ascending or descending order:
•   Select * from Organisation Order by Org_Code desc
• Syntax: Select * from [table] Order by [column name] desc/asc
•   Select * from Organisation where Org_Code>100 order by Org_Name desc

•  Finding rows with value as null for a column
•   Select * from Organisation where Org_type is null
•  Syntax: Select * from [table] where [column name]  is null

•        Finding rows with value as not null for a column
•         Select * from Organisation where Org_type is not null
•       Syntax: Select * from [table] where [column name]  is not null

•   Finding Count of records matching the SQL query
•    Select Count(1) from Organisation where Org_Type = ‘Auto’ Or Org_Code = 201
• Using count(1) makes the query more faster compared to count(*)

•    The LIKE operator is used to search for a specified pattern in a column.
• Select * from Organisation where Org_Name like ‘%test%’.
• In the example % acts as wildcard to search for pattern which test displayed anywhere in the Org_Name.
• Other examples include:
• Select * from Organisation Where Org_Name  like '_ estOrg2'; (wildcard for one character)
• Select * from Organisation Where OrgName  like '[tfd]%’; (wildcard for all org_name starting with t, f, or d)