1. Common
SQL Programming
Mistakes

2. Who am I??
Kevin G. Boles SQL Server Consultant “The Wolfe” for the Relational Engine Indicium Resources, Inc. AIM/Twitter: TheSQLGuru Gtalk/GMail: MCITP, MCT, MVP , yada-yada

3. What do I do?
World-class Better than average relational engine expert 
Almost 45,000 man-hours invested in SQL Server
Absolutely LOVE doing targeted performance analysis and tuning work
VERY good at making others better at interacting with SQL Server
Couldn’t SSAS my way out of a paper bag
… with both ends open! 

4. Why learn from others mistakes?
Smart people Learn from their mistakes. Wise people Learn from other peoples mistakes.

5. Agenda Why do we make these mistakes? Common SQL programming mistakes:
NULLs and the NOT IN predicate
Functions on indexed columns in predicates
Incorrect subquery column
Data type mismatch in predicates
Predicate evaluation order
Outer joins and placement of predicates
Subqueries that return more than one value
Use of SELECT *
Scalar user-defined functions
Overuse of cursors
Triggers
Other mistakes

6. Why do we make these mistakes?
Coming from different background (Java, C, C++, etc.)
Inherited responsibilities
Learning the wrong way (or from incorrect sources)
Bad practices in team collaboration
Incorrect design
Not testing exception cases
Overworking (not enough caffeine)
On the wrong bus
Never engaged TheSQLGuru as a consultant/mentor!! 

7. NULLs and the NOT IN predicate/*
Colors:
color
Black
Blue
Green
Red
Products:
sku product_description color
1 Ball Red
2 Bike Blue
3 Tent NULL */
SELECT C.color
FROM Colors AS C
WHERE C.color NOT IN (SELECT P.color
FROM Products AS P);

8. Functions on indexed columns in predicates
SELECT customer_name
FROM Customers
WHERE LEFT(customer_name, 1) = 'L';
SELECT SUM(sale_amount) AS total_sales
FROM Sales
WHERE DATEPART(YEAR, sale_date) = 2009
AND DATEPART(MONTH, sale_date) = 1;
/* Exception: SQL Server */
WHERE CAST(sale_date AS DATE) = ' ';

9. Incorrect subquery column
/* Left to the inquisitive attendee 
ALWAYS USE ALIASES FOR EVERYTHING!!
Sales:
sale_date sale_amount
Calendar:
calendar_date holiday_name
New Year's Day
NULL
NULL
NULL
NULL */
SELECT sale_date, sale_amount
FROM Sales AS S
WHERE sale_date IN (SELECT sale_date
FROM Calendar AS C
WHERE holiday_name IS NOT NULL);

10. Data type mismatch in predicates
CREATE TABLE Customers (
customer_nbr INT NOT NULL PRIMARY KEY,
first_name VARCHAR(35) NOT NULL,
last_name VARCHAR(35) NOT NULL);
SELECT first_name, last_name
FROM Customers
WHERE last_name = N'Brown'; /*
StmtText
|--Clustered Index Scan(OBJECT:( [dbo].[Customers].[PK_Customer]),
WHERE:(CONVERT_IMPLICIT(nvarchar(35), */

11. Data type mismatch in predicates (solution)
SELECT first_name, last_name
FROM Customers
WHERE last_name = 'Brown';
BEWARE calling application for this. Some connection mechanisms if you use defaults (ADO.NET) will declare vars sent to SQL Server as Nvarchar. I just came across 3rd party app that did this and got order of magnitude plus performance gain across the board and HUGE increases in concurrency from explicitly declaring proper datatypes in data access layer!!!

12. Predicate evaluation order/*
Accounts:
account_nbr account_type account_reference
Personal abc
Business Basic 101
Personal def
Business Plus 5 */
SELECT account_nbr, account_reference AS account_ref_nbr
FROM Accounts
WHERE account_type LIKE 'Business%'
AND CAST(account_reference AS INT) > 20;
SELECT account_nbr, account_ref_nbr
FROM (SELECT account_nbr,
CAST(account_reference AS INT) AS account_ref_nbr
WHERE account_type LIKE 'Business%') AS A
WHERE account_ref_nbr > 20;

13. Outer joins and placement of predicates
SELECT C.customer_name, SUM(COALESCE(O.order_amt, 0)) AS total_2009
FROM Customers AS C
LEFT OUTER JOIN Orders AS O
ON C.customer_nbr = O.customer_nbr
WHERE O.order_date >= ' '
GROUP BY C.customer_name;
SELECT C.customer_name, O.order_amt, D.qty
INNER JOIN OrderDetails AS D
ON D.order_nbr = O.order_nbr
AND D.sku = 101;

14. Subqueries that return more than one value -- Left to the inquisitive attendee 
SELECT sku, product_description,
(SELECT plant_nbr
FROM ProductPlants AS B
WHERE B.sku = A.sku) AS plant_nbr
FROM Products AS A;
SELECT A.sku, A.product_description, B.plant_nbr
FROM Products AS A
JOIN ProductPlants AS B
ON A.sku = B.sku
WHERE B.sku = (SELECT C.sku
FROM NewProducts AS C
WHERE C.product_description LIKE '%guitar%');

15. Use of SELECT. ALWAYS use explicit column names
Use of SELECT * ALWAYS use explicit column names! Easy with code generators now!
Talk on view error, show performance issue
INSERT INTO PlasticProducts
SELECT *
FROM Products
WHERE material_type = 'plastic';
CREATE VIEW MetalProducts AS
WHERE material_type = 'metal';

16. Scalar user-defined functions
CREATE FUNCTION [dbo].[fn_TotalPrice] int)
RETURNS money WITH SCHEMABINDING AS
BEGIN
money
= SUM(UnitPrice * Quantity) FROM dbo.[Order Details] WHERE OrderID
END

17. Overuse of cursors (and WHILE, recursive CTEs)
/* Left to the inquisitive attendee :)
almost NEVER a need for these – running totals maybe (DENALI WIN!!) */
DECLARE PriceUpdates
CURSOR LOCAL
FORWARD_ONLY
STATIC
READ_ONLY
FOR SELECT sku, price
FROM NewPrices;
OPEN PriceUpdates;
FETCH NEXT FROM PriceUpdates
@price;
WHILE = 0
BEGIN
UPDATE ProductPrices
SET price
effective_start_date = CURRENT_TIMESTAMP
WHERE sku
END

18. Triggers and Multiple Rows
Triggers do NOT fire once per row!! Body of trigger: varchar(10) = = myChar FROM INSERTED INSERT AuditTable

19. IS NULL OR BADNESS
CREATE PROC badIsNULLORProc int) AS SET NOCOUNT ON SELECT a, b, c FROM dbo.KGBIsNULLORTest WHERE = a IS NULL) AND = b IS NULL) RETURN

20. Other mistakes SQL-89 vs. SLQ-92 join syntax Date ranges Date formats
Missing correlation in subqueries
Data type precedence in expressions
Integer division
MONEY vs. DECIMAL/float (big deal in large DWs)
Declaring VARCHAR without length
Division by zero handling
“Ordered” views
NULL does not equal NULL or other values
Nondeterministic updates
Dynamic searches
Arithmetic operator precedence
Other…

21. Questions and comments
The trouble with people is not that they don't know
but that they know so much that ain't so.
Josh Billings
Kevin G. Boles
@TheSQLGuru
Slide Deck by:
Plamen Ratchev
blog :