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What is the FROM Clause?

The FROM clause in SQL is used to specify the table or tables from which to retrieve data. It is a mandatory clause in a SELECT statement (unless you’re using a query without a table, like SELECT 1+1). The FROM clause tells the database which table(s) to query for the data.

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Key Points

  1. Table Specification: The FROM clause specifies the table(s) from which data is retrieved.
  2. Single or Multiple Tables: You can query data from a single table or join multiple tables using the FROM clause.
  3. Alias Support: You can assign aliases to tables for easier reference in the query.
  4. Subqueries: The FROM clause can also include subqueries, allowing you to query data from the result of another query.

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Syntax

SELECT column1, column2, ...
FROM table_name;
  • column1, column2, ...: The columns you want to retrieve.
  • table_name: The name of the table from which to retrieve data.

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Example

Suppose you have a table named Employees that stores employee details.

Table: Employees

EmployeeIDNameCityState
1AnandChennaiTamil Nadu
2BalaCoimbatoreTamil Nadu
3KavithaKaraikalPuducherry
4RajMaduraiTamil Nadu
5KumarTrichyTamil Nadu
  1. To retrieve the names and cities of all employees:
SELECT Name, City
FROM Employees;
NameCity
AnandChennai
BalaCoimbatore
KavithaKaraikal
RajMadurai
KumarTrichy

Suppose you have a table named Students that stores student details, and you want to retrieve the names of students from Tamil Nadu.

Table: Students

StudentIDNameCityState
1RamChennaiTamil Nadu
2KarthikCoimbatoreTamil Nadu
3DavidBangaloreKarnataka
4KannanKaraikalPuducherry
5SivaMaduraiTamil Nadu
SELECT Name
FROM Students
WHERE State = 'Tamil Nadu';
Name
Ram
Karthik
Siva

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Using the FROM Clause with Multiple Tables

The FROM clause can also be used to retrieve data from multiple tables using joins. For example, suppose you have another table named Departments.

Table: Departments

DepartmentIDEmployeeIDDepartmentName
1011HR
1022Finance
1033IT
1044Marketing
1055Sales

To retrieve the employee names and their corresponding department names:

SELECT Employees.Name, Departments.DepartmentName
FROM Employees
JOIN Departments ON Employees.EmployeeID = Departments.EmployeeID;
NameDepartmentName
AnandHR
BalaFinance
KavithaIT
RajMarketing
KumarSales

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Using Table Aliases in the FROM Clause

Table aliases can be used to simplify queries, especially when dealing with long table names or multiple tables.

SELECT e.Name, d.DepartmentName
FROM Employees AS e
JOIN Departments AS d ON e.EmployeeID = d.EmployeeID;
NameDepartmentName
AnandHR
BalaFinance
KavithaIT
RajMarketing
KumarSales

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Using Subqueries in the FROM Clause

The FROM clause can also include subqueries. For example, suppose you want to retrieve employees who work in the IT department.

SELECT Name
FROM (
    SELECT Employees.Name
    FROM Employees
    JOIN Departments ON Employees.EmployeeID = Departments.EmployeeID
    WHERE Departments.DepartmentName = 'IT'
) AS IT_Employees;
Name
Kavitha

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Key Takeaways

  1. The FROM clause specifies the table(s) from which data is retrieved.
  2. It is mandatory in a SELECT statement (unless querying without a table).
  3. You can query data from a single table, multiple tables (using joins), or subqueries.
  4. Table aliases can simplify queries, especially with long table names or multiple tables.
  5. The FROM clause is essential for retrieving and combining data from one or more sources.