SELECT—This is the primary clause of the SELECT statement and is absolutely required. You use it to specify the columns you want in the result set of your query. The columns themselves are drawn from the table or view you specify in the FROM clause. You can also use aggregate functions, such as Sum(HoursWorked), or mathematical expressions, such as Quantity * Price, in this clause.
FROM—This is the second most important clause in the SELECT statement and is also required. You use the FROM clause to specify the tables or views from which to draw the columns you've listed in the SELECT clause. You can use this clause in more complex ways.
WHERE—This is an optional clause that you use to filter the rows returned by the FROM clause. The WHERE keyword is followed by an expression, technically known as a predicate, that evaluates to true, false, or unknown. You can test the expression by using standard comparison operators, Boolean operators, or special operators.
GROUP BY—When you use aggregate functions in the SELECT clause to produce summary information, you use the GROUP BY clause to divide the information into distinct groups. Your database system uses any column or list of columns following the GROUP BY keywords as grouping columns. The GROUP BY clause is optional,
HAVING—The HAVING clause filters the result of aggregate functions in grouped information. It is similar to the WHERE clause in that the HAVING keyword is followed by an expression that evaluates to true, false, or unknown. You can test the expression by using standard comparison operators, Boolean operators, or special operators. HAVING is also an optional clause.