How to write a SQL - Query

REFERENCE : http://www.ifadey.com/2010/11/best-way-to-write-sql-query/

 

Clause Order

Thinking and writing SQL query clauses in particular order (in which database engine executes the query) matters a lot because problem (getting required data) gets much simplified by creating query in such an order. Actually the problem is automatically divided and simplified when writing query in execution order. Let’s take a look how the database engine executes the query.

1. The Source of Data

The first clause which gets executed is FROM clause. It means the first step when you write a query is to know FROM which table(s) you want to get the data. If the required data is in multiple tables, then think about the JOINs in first step. When FROM clause gets executed, all rows are fetched from the tables specified. Let’s call them Individual Rows.
Click here to read more about SQL JOINs.

2. Filter Individual Rows

Second step is to think about WHERE clause which is used to filter Individual Rows on particular condition(s).

3. Grouping of Individual Rows

GROUP BY is the third clause which gets executed. So think about it in the third step when writing a query. This clause group Individual Rows (fetched using FROM and filtered using WHERE) on the basis and order of columns specified.
Click here to read more about SQL GROUP BY.

4. Filter Group Rows

HAVING clause is used to filter Group Rows based on the condition(s) specified in it. It’s fourth one which gets executed.

5. Filter Columns

The fifth clause which gets executed is SELECT. This clause is used to filter columns which are not required.

6. Ordering of Rows

The final step in execution of query is to order the rows based on specified column(s).