SQL Query and the Order of its syntax

Week 7 - SQL

Created: 2022-08-02
Tags: #fleeting

That doesn't mean that every parts must be included.
Some parts can be omitted but the order of the syntax must be followed so.

SELECT DISTINCT column, AGG_FUNC(column_or_expression), …
FROM mytable
    JOIN another_table
      ON mytable.column = another_table.column
    WHERE constraint_expression
    GROUP BY column
    HAVING constraint_expression
    ORDER BY column ASC/DESC
    LIMIT count OFFSET COUNT;

1. FROM and JOINs

The FROM clause, and subsequent JOINs are first executed to determine the total working set of data that is being queried. This includes subqueries in this clause, and can cause temporary tables to be created under the hood containing all the columns and rows of the tables being joined.

2. WHERE

Once we have the total working set of data, the first-pass WHERE constraints are applied to the individual rows, and rows that do not satisfy the constraint are discarded. Each of the constraints can only access columns directly from the tables requested in the FROM clause. Aliases in the SELECT part of the query are not accessible in most databases since they may include expressions dependent on parts of the query that have not yet executed.

3. GROUP BY

The remaining rows after the WHERE constraints are applied are then grouped based on common values in the column specified in the GROUP BY clause. As a result of the grouping, there will only be as many rows as there are unique values in that column. Implicitly, this means that you should only need to use this when you have aggregate functions in your query.

4. HAVING

If the query has a GROUP BY clause, then the constraints in the HAVING clause are then applied to the grouped rows, discard the grouped rows that don't satisfy the constraint. Like the WHERE clause, aliases are also not accessible from this step in most databases.

5. SELECT

Any expressions in the SELECT part of the query are finally computed.

6. DISTINCT

Of the remaining rows, rows with duplicate values in the column marked as DISTINCT will be discarded.

7. ORDER BY

If an order is specified by the ORDER BY clause, the rows are then sorted by the specified data in either ascending or descending order. Since all the expressions in the SELECT part of the query have been computed, you can reference aliases in this clause.

ORDER BY rating DESC, title ASC;

First it arranges rows in rating descending order
Second, those ordered by rating will remain untouched
Only the same values with rating will be sorted with title 

{'8.8', 'Inception'}
{'8.3', 'Toy Story 3'}
{'8.1', 'How to Train Your Dragon'}
{'8.1', 'Shutter Island'}

8. LIMIT / OFFSET

Finally, the rows that fall outside the range specified by the LIMIT and OFFSET are discarded, leaving the final set of rows to be returned from the query.

Conclusion

Not every query needs to have all the parts we listed above,
but a part of why SQL is so flexible is that
it allows developers and data analysts to quickly manipulate data
without having to write additional code, all just by using the above clauses.

References

Interactive Graph
Table Of Contents
SQL Query and the Order of its syntax
1. FROM and JOINs
2. WHERE
3. GROUP BY
4. HAVING
5. SELECT
6. DISTINCT
7. ORDER BY
8. LIMIT / OFFSET
Conclusion
References