1. Steve Coleman UTOUG Fall Symposium October 26, 2016
SQL The Director’s Cut
Steve Coleman
UTOUG Fall Symposium
October 26, 2016

2. Background Database developer since 2006 PL/SQL Developer
Quattro Pro
Excel
SQL Server/Access
Oracle
PL/SQL Developer
Data Detective
10+ years of OJT

3. Mitsakes, I’ve made a few
Let’s go through a couple of common, easy-to-make mistakes

4. Understand your tools What SQL IS Set-based language Relational data
ANSI compliant (SQL 92)
What SQL IS NOT
Object-oriented
Procedural
Unstructured
ANSI compliant (SQL 92)
SQL is a query language, not a procedural programming language.

5. Logical Query Processing
What you write
What actually happens
5 - SELECT
1 - FROM
1 - FROM
2 - WHERE
2 - WHERE
3 - GROUP BY
3 - GROUP BY
4 - HAVING
4 - HAVING
5 - SELECT
6 - ORDER BY
6 - ORDER BY

6. Survey says…
If you know the data you may be lulled into a sense of false security because it just so happens that King, Kochhar, and De Haan are the top 3 salaries. But remember that SELECT is executed before ORDER BY and ROWNUM is assigned during the SELECT phase. You can not assume an order to a query. If you need order you must ORDER BY. And if you need the top n results you must order in an inner query.

7. LEFT INNER JOIN SELECT e.first_name ,e.last_name ,e.hire_date
,e.job_id
,h.*
FROM hr.employees e
LEFT OUTER JOIN hr.job_history h ON h.employee_id = e.employee_id
There are 107 total employees. So 110 rows means that some employees have more than one history record.
WHERE h.start_date > TO_DATE('13-JAN-2001', 'dd-MON-yyyy');

8. Virtually the same … FROM employees
EMPLOYEE_ID
FIRST_NAME
LAST_NAME …
100
Steven
King
SKING
101
Neena
Kochhar
NKOCHHAR
102
Lex
De Haan
LDEHAAN
LEFT OUTER JOIN job_history ON employee_id = employee_id
EMPLOYEE_ID
FIRST_NAME
LAST_NAME …
START_DATE
JOB_ID…
100
Steven
King
SKING
(null)
101
Neena
Kochhar
NKOCHHAR
21-SEP-97
AC_ACCOUNT
28-OCT-01
AC_MGR
102
Lex
De Haan
LDEHAAN
13-JAN-01
IT_PROG

9. LEFT OUTER JOIN SELECT e.first_name ,e.last_name ,e.hire_date
,e.job_id
,h.*
FROM hr.employees e
LEFT OUTER JOIN hr.job_history h ON h.employee_id = e.employee_id
AND h.start_date > TO_DATE('13-JAN-2001', 'dd-MON-yyyy');
Remember that the WHERE clause is processed after the FROM and JOINS. In the WHERE clause only records meeting the criteria will be returned. Employees with no history don’t match the WHERE criteria, so the record is thrown out. All filters on data in the RIGHT table of a LEFT JOIN must be included in the ON clause

10. A parenthetical example
SELECT *
FROM hr.employees
WHERE department_id IN (30,80)
AND job_id = 'PU_MAN'
OR (job_id like '%MAN' AND commission_pct >= .4)
AND job_id LIKE '%MAN'

11. A parenthetical example (cont.)
SELECT *
FROM hr.employees
WHERE department_id IN (30,80)
AND ( job_id = 'PU_MAN'
OR (job_id like '%MAN' AND commission_pct >= .4))

12. What’s in an Alias? 1. Does the departments table have employee_id?
No. Okay, so what about the employees table?
Yes. Great, let’s just use that
4. So we’re left with employee_id = employee_id

13. ANSI Isolation Levels Isolation Level Dirty Read Non-repeatable
Phantom
READ UNCOMMITTED
Yes
READ COMMITTED No
REPEATABLE READ
SERIALIZABLE
Dirty Read: uncommitted data
Transaction isolation levels are defined in terms of 3 phenomena that can happen when reading data in a database where writes are occurring concurrently. Each isolation level is defined by which of these phenomena are allowed to occur.
Non-repeatable Read: A record read at time T1 may give different results when read at T2
Phantom Read: A query ran at time T1 may give different results at T2 because more data
satisfies the query criteria than before

14. Database Isolation Levels
Oracle
SQL SERVER
READ UNCOMMITTED
READ COMMITTED (default)
REPEATABLE READ
SNAPSHOT (2005+)
SERIALIZABLE
READ-ONLY
You need to understand how your tool handles concurrency so you don’t make mistakes.
The purpose of READ UNCOMMITTED is to allow for a non-blocking read, which Oracle already does.

15. To COMMIT or not COMMIT
The database doesn’t define transaction boundaries. You do that, whether you realize it or not
Session 1
Session 2
CREATE TABLE table_a (col_a integer);
INSERT INTO table_a VALUES(1);
ALTER TABLE table_a ADD (col_b VARCHAR2(1));
SELECT COUNT(*) FROM table_a;
UPDATE table_a SET col_b = ‘A’ WHERE col_a = 1;

16. Double the fun SELECT COUNT(*) Try this instead INTO l_counter
FROM tables …
WHERE …
IF l_counter > 0 THEN
UPDATE … …
Try this instead
UPDATE …
WHERE … …
l_counter := SQL%ROWCOUNT
Table is hit twice for no reason

17. Greece …in the details The Hellenic Republic
Some mistakes happen long before code is written
More commonly known as….
The
Hellenic
Republic
Greece

18. A wrinkle in time...queries
Effective dates
Start Date
End Date
Does it mean that it ENDS on this date, or that it ENDED as of this date
What time?
Make a plan to write date-range queries consistently

19. A letter to the editor EDIT PEER REVIEW
Take a mental break between writing and editing
Be critical
Do I even need to do this at all
PEER REVIEW
Don’t over-explain
Provide some context, but don’t be too helpful
If you can’t give a solid answer to questions, be worried
Don’t assume the author is competent

20. Let’s review

21. SQL – The Director’s Cut
Know your tools and environment
Know your business and requirements
Edit your own code
Edit someone else’s code
Great code has as much to do with what you leave out as it does what you put in