When does WHERE filtering occur? [duplicate]

Answer 1

Where clause will be applied before join i.e. it will return all the rows of A and corresponding rows from B and C depend upon join condition and for type = 1

Answer 2

Per MSDN.
Check the section titled Logical Processing Order of the SELECT statement

1. FROM
2. ON
3. JOIN
4. WHERE
5. GROUP BY
6. WITH CUBE or WITH ROLLUP
7. HAVING
8. SELECT
9. DISTINCT
10. ORDER BY
11. TOP

There is an important note at the end of the paragraph preceding this list.

Note that the actual physical execution of the statement is determined by the query processor and the order may vary from this list.

Answer 3

To start with, below is the SQL Order of Operations:

• FROM clause
• WHERE clause
• GROUP BY clause
• HAVING clause
• SELECT clause
• ORDER BY clause

In a simple query, the filtering happens after the FROM clause (joins are found on this part). What your querty above does is it primarily joins the tables with their linking columns that defines their relationship. After the records has been set (the result of joins) the WHERE clause then takes place to filter out Type where is is equal to 1.

---

Here's another example of using LEFT JOIN,

First Query:

SELECT  A.ID,
        A.Name,
        A.Type,
        B.FirstName,
        B.LastName,
        B.DateOfBirth
FROM    A
        LEFT JOIN B 
            ON  A.ContactID = B.ID AND
                B.LastName = 'Michaels'

vs Second Query:

SELECT  A.ID,
        A.Name,
        A.Type,
        B.FirstName,
        B.LastName,
        B.DateOfBirth
FROM    A
        LEFT JOIN B ON  A.ContactID = B.ID
WHERE   B.LastName = 'Michaels'

The first query returns ALL the records from table A. What B.LastName = 'Michaels' does is before the table B will be join with table A, it filters out all the records where the LastName is equal to Michaels. So the records from table A which do not have matches on the filtered records on Table B will have NULL values on the columns from Table B.

The second query will not yield the same result with the first query and performs exactly the same with INNER JOIN because after the records has been joined, another filtering will be executed on the result and takes only records where the LastName is equal to Michaels.

Answer 4

Logically - after joins, physically - it's up to optimizer.

Answer 5

To answer this, you really need to look at the execution plan. In the case you mention, where you have a searchable argument (a SARG) in the where clause, the filter would most likely be applied with your access method, i.e. index, table scan, etc., before the join.

Use

Query Menu -> Include Actual Execution Plan

or

Query Menu -> Display Estimated Execution Plan

to take a look.