Sql-server – join table and count them SQL based on segment

The logical processing of a query is on MSDN (written by Microsoft SQL Server team, not 3rd parties)

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

A derived table follows this, then the outer query does it again etc etc

This is logical though: not actual. No matter how SQL Server actually does it, these semantics are honoured to the letter. The "actual" is determined by the Query Optimiser (QO) and you avoid the intermediate Cartesion product you mentioned.

It's worth mentioning that SQL is declarative: you say "what" not "how" like you would for a procedural/imperative programming (Java, .net). So saying "this happens before that" is wrong in many cases (eg assumption of short circuits or L-to-R WHERE order)

In your case above, the QO will generate the same plan no matter how it is structured because it is a simple query.

However, the QO is cost based and for a complex query it may take 2 weeks to generate the ideal plan. So it does "good enough" which actually isn't.

So your first case may help the optimiser find a better plan because the logical processing order is different for the 2 queries. But it may not.

I have used this trick on SQL Server 2000 to get 60x speed performance improvement on reporting queries. As the QO improves version to version it gets better at working these things out.

And the book you mentioned: there is some dispute over it
See SO and the subsequent links: https://stackoverflow.com/q/3270338/27535

When you join to tables together you need to think of it comparing each row from the first table to each row of the second table and then returning the ones that match based on the join condition. The join condition is specified in the ON clause.

In almost all cases you want to reference both tables in the ON clause. Let's look at your example in SQL Fiddle

MySQL 5.5.32 Schema Setup:

CREATE TABLE Question (
  QID INT,
  QTxt VARCHAR(150),
  QOrder INT);

CREATE TABLE User (
  UID INT,
  UserName VARCHAR(150));

CREATE TABLE ANSWER (
  AID INT,
  ATxt VARCHAR(150),  
  QID INT,
  USERID INT);




INSERT INTO User VALUES
 (1, 'Alex'),
 (2, 'Sarah'),
 (3, 'Tom');

INSERT INTO Question VALUES
 (1, 'Age', 1),
 (2, 'Gender', 2),
 (3, 'Weight',3),
 (4, 'Height',4);

INSERT INTO Answer VALUES
 (1, '30', 1 , 1),
 (2, '27', 1,2),
 (3, '30', 1,3),
 (4, 'M', 2,1),
 (5, 'F', 2,2),
 (6, 'M', 2,3),
 (7, '61', 4,1),
 (8, '165', 3,1);

The ON condition in your case is Answer.QID = Question.QID:

Query 1:

SELECT *
  FROM Answer
  JOIN Question
    ON Answer.QID = Question.QID
 WHERE Answer.UserId = 3

Results:

| AID | ATXT | QID | USERID |   QTXT | QORDER |
|-----|------|-----|--------|--------|--------|
|   3 |   30 |   1 |      3 |    Age |      1 |
|   6 |    M |   2 |      3 | Gender |      2 |

You can use aliases to shorten that query a little:

Query 2:

SELECT *
  FROM Answer AS A
  JOIN Question AS Q
    ON A.QID = Q.QID
 WHERE A.UserId = 3

Results:

| AID | ATXT | QID | USERID |   QTXT | QORDER |
|-----|------|-----|--------|--------|--------|
|   3 |   30 |   1 |      3 |    Age |      1 |
|   6 |    M |   2 |      3 | Gender |      2 |

The same way you can now also join in the User table:

Query 3:

SELECT *
  FROM Answer AS A
  JOIN Question AS Q
    ON A.QID = Q.QID
  JOIN User AS U
    ON A.UserId = U.UID
 WHERE U.UserName = 'Tom'

Results:

| AID | ATXT | QID | USERID |   QTXT | QORDER | UID | USERNAME |
|-----|------|-----|--------|--------|--------|-----|----------|
|   3 |   30 |   1 |      3 |    Age |      1 |   3 |      Tom |
|   6 |    M |   2 |      3 | Gender |      2 |   3 |      Tom |

For more information on JOINs check out my JOIN series here: A Join a Day

It is written about Microsoft SQL Server, but most of it works in other database management system too.