SQL - Clauses | Where | TOP, Limit, Fetch First or Rownum Clause | Group BY | Having | Order BY

Where Clause:

The WHERE clause is used to filter records.
It is used to extract only those records that fulfill a specified condition.

Syntax

SELECT column1, column2, ... FROM table_name
WHERE condition;

Example:
:

Select all customers from Mexico:
WHERE Country='Mexico';

SELECT * FROM Customers
WHERE CustomerID=1;

SQL TOP, LIMIT, FETCH FIRST or ROWNUM Clause

The SELECT TOP clause is used to specify the number of records to return.

The SELECT TOP clause is useful on large tables with thousands of records. Returning a large number of records can impact performance.

Note: Not all database systems support the SELECT TOP clause. MySQL supports the LIMIT clause to select a limited number of records, while Oracle uses FETCH FIRST n ROWS ONLY and ROWNUM.

SELECT TOP 3 * FROM Customers;

SELECT TOP 50 PERCENT * FROM Customers;

SELECT * FROM Customers
LIMIT 3;

SELECT * FROM Customers
FETCH FIRST 3 ROWS ONLY;

SELECT * FROM Customers
FETCH FIRST 50 PERCENT ROWS ONLY;

Limit Clause:

Sets an upper limit on number of rows(tuples) to be returned.

Syntax:

Select col1,col2 from table_name

Limit number;

Example:

Select * from student 

Limit 3;

Order by clause:

The ORDER BY keyword is used to sort the result-set in ascending or descending order.

The ORDER BY keyword sorts the records in ascending order by default. To sort the records in descending order, use the DESC keyword.

SELECT column1, column2, ...
FROM table_name
ORDER BY column1, column2, ... ASC|DESC;

SELECT * FROM Products
ORDER BY Price;

The following SQL statement selects all customers from the "Customers" table, sorted by the "Country" and the "CustomerName" column. This means that it orders by Country, but if some rows have the same Country, it orders them by CustomerName

SELECT * FROM Customers
ORDER BY Country, CustomerName;

Group by clause:

The GROUP BY statement groups rows that have the same values into summary rows, like "find the number of customers in each country".

The GROUP BY statement is often used with aggregate functions (COUNT(), MAX(), MIN(), SUM(), AVG()) to group the result-set by one or more columns.

SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
ORDER BY column_name(s);

SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country;

Having clause:

The HAVING clause was added to SQL because the WHERE keyword cannot be used with aggregate functions.

SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
HAVING condition
ORDER BY column_name(s);

The following SQL statement lists the number of customers in each country. Only include countries with more than 5 customers:

SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country
HAVING COUNT(CustomerID) > 5;

General order of commands:

Select columns

From table_name

Where condition

Group by columns

Having condition

Order by columns ASC;