1. T-SQL : Bad Habits to Kick Aaron Bertrand SQL Sentry, Inc.

2. Who is Aaron Bertrand? Senior Consultant at SQL Sentry www.sqlsentry.net abertrand@sqlsentry.net Microsoft MVP since 1997-98 Blog: www.sqlblog.comwww.sqlblog.com Twitter: @AaronBertrand@AaronBertrand 2 AD-204 | T-SQL : Bad Habits to Kick

3. Agenda 12 simple slides with one common goal: Improving at least one bad habit. These are mostly just opinions; No right or wrong answer. 3 AD-204 | T-SQL : Bad Habits to Kick

4. 1. SELECT * / omitting column list Needless lookups/scans, I/O, network load Predictability / change management Today’s tools negate carpal tunnel excuse AD-204 | T-SQL : Bad Habits to Kick 4

5. 2. Declaring variables without length Guess the results: DECLARE @x VARCHAR = 'foo'; SELECT @x; SELECT CONVERT(VARCHAR, 'foo'); AD-204 | T-SQL : Bad Habits to Kick 5

6. 3. Choosing the wrong data type Don’t choose: String/numeric types for date/time data TIME in place of an interval DATETIME if DATE/SMALLDATETIME will do NVARCHAR(MAX) for URL, zip, phone, e- mail VARCHAR for proper names AD-204 | T-SQL : Bad Habits to Kick 6

7. 4. Not using schema prefix Being explicit prevents confusion or worse Object resolution works harder without it Leads to multiple cached plans for same query Even if all objects belong to dbo, specify Eventually, you or 3 rd party will use schemas AD-204 | T-SQL : Bad Habits to Kick 7

8. 5. Using inconsistent naming conventions Examples I’ve seen in a single system: GetCustomerDetails Customer_Update Create_Customer Styles vary – even your own changes over time The convention you choose isn’t the point AD-204 | T-SQL : Bad Habits to Kick 8

9. 6. Using loops to populate large tables WHILE…INSERT 1,000,000 times is log intensive Much better constructs: Numbers table Recursive CTEs Cross joins from catalog views If you must use a loop Batch by committing every rows AD-204 | T-SQL : Bad Habits to Kick 9

10. 7. Mishandling date range queries Avoid non-sargeable clauses that come from: YEAR() and other functions against columns CONVERT() on both sides of clauses BETWEEN is ok for DATE but not DATETIME Do not try to calculate “end of today”: Use >= today AND < tomorrow AD-204 | T-SQL : Bad Habits to Kick 10

11. 8. Using SELECT/RETURN vs. OUTPUT In general… SELECT is for multiple rows/columns OUTPUT is for limited number of scalar values RETURN is for status/error codes, NOT DATA! AD-204 | T-SQL : Bad Habits to Kick 11

12. 9. Using old-style joins Do not use old-style inner joins (FROM x, y, z) Easy to accidentally derive Cartesian product Not deprecated, but not recommended either Do not use old-style outer joins (*= / =*) Deprecated syntax Unpredictable results AD-204 | T-SQL : Bad Habits to Kick 12

13. 10. Using cursors with default options Cursors are often bad, but not evil Avoid heavy locking behavior - my syntax: DECLARE c CURSOR LOCAL STATIC READ_ONLY FORWARD_ONLY FOR … AD-204 | T-SQL : Bad Habits to Kick 13

14. 11. Using ORDER BY [ordinal] Underlying structure/query changes OK habit for ad hoc stuff, not production code Keystrokes are only downside to being explicit IntelliSense / 3 rd party tools negate this anyway AD-204 | T-SQL : Bad Habits to Kick 14

15. 12. Assuming ORDER without ORDER BY Popular myth: “table has natural order” Without ORDER BY, there is no guaranteed order TOP unfortunately has two meanings: 1.Which rows to include 2.How to order them To separate, use a CTE or nested subquery AD-204 | T-SQL : Bad Habits to Kick 15

16. 13 -> …plenty of others… Search for “bad habits to kick” at http://sqlblog.com/ AD-204 | T-SQL : Bad Habits to Kick 16 ∞