1. There’s a particular style to it…
Thinking in SQL
There’s a particular style to it…
Rob Hatton

2. SQL is not a complete language
No way to build a user interface
No way to read or write to a file
Can’t print
Doesn’t even know about variables
It’s awesome at retrieving data!

3. SQL Server has 2 languages
ANSI SQL
Based on Sets
TSQL
We’ll be talking about ANSI SQL

4. SQL is a different kind of language
Most languages are procedural
Like telling a story
It’s the reason for TSQL
SQL is a declarative language
Like painting a picture
The complete solution does both
It’s a comic book
Sets are key to understanding SQL

5. Types Of SQL Statements
Data Definition Language (DDL)
Create (Table, View, Proc)
Alter (Table, View, Proc)
Drop (Table, View, Proc)
Data Manipulation Language (DML)
Select
Insert
Delete
Update
Database definition is stored in a database. DDL manipulates the definition.
We’re not going to focus on DDL.

6. Select Syntax SELECT field_list [ FROM set_source ] (includes joins)
[ WHERE search_expression ] (record based)
[ GROUP BY group_expression ]
[ HAVING aggregate_expression ]
[ ORDER BY order_expression [ ASC | DESC ] ]
Where clause logic is record based, not set based!
‘Where eyeColor = ‘blue’ and eyeColor = ‘green’ returns no records.
‘Group by’ requires an aggregate function in select list.
‘Having’ only works with group by.
Where clause logic is record based, not set based!
‘Where eyeColor = ‘blue’ and eyeColor = ‘green’ returns no records.
‘Group by’ requires an aggregate function in select list.
‘Having’ only works with group by.

7. Aliasing select fis.orderDateKey , max(fis.salesAmount) as maxAmt
from factInternetSales fis
group by fis.orderDateKey
Table Alias: fis
Field Alias: maxAmt

8. Select examples * select 'just some text' as txt
select getdate() as dt
select City from DimGeography
select lastName from vTargetMail
select color from (select 'blue' as color) as colors
Select * is bad, it eats memory!
use [AdventureWorksDW2012] go
select 'just some text' as txt
select getdate() as dt
select City from DimGeography
select lastName from vTargetMail
select color from (select 'blue' as color) as colors *

9. Functions and Expressions
UPPER() - Converts a field to upper case
SUBSTR() - Returns part of a character expression
LTRIM() – Remove spaces from left end of text field
LEN() - Returns the length of a text field
GETDATE() - Returns the system date and time
CHARINDEX() – Returns start of string in another string
Case When
Beware of misuse

10. Sets Tables Views Sub-queries Common Table Expressions Regular tables
Temp tables
Table objects
Function return
Views
Sub-queries
Common Table Expressions

11. Comparison Operators = < <= > <> ! (not) Between ExistsIn
Is [not] null
Remember, they are record based not set based!

12. Joins * Types Joins work with any comparison operator!
Inner
Outer
Left
Right
Cross
Full
Self
Exists
Joins work with any comparison operator!
Joins aren’t as much of a problem as data size!
--Joins
use Fruit go
select f.fruitName, c.color
from fruit f
inner join colors c
on f.colorKey = c.colorKey
from colors c
left outer join fruit f
right outer join fruit f
--Self join
use [AdventureWorksDW2012]
select c.AccountDescription, p.AccountDescription
from DimAccount c
left outer join DimAccount p
on p.ParentAccountKey = c.AccountKey
order by c.accountKey
--exists
select e.lastName
from DimEmployee e
where not exists
(select * from FactResellerSales frs
where frs.EmployeeKey = e.EmployeeKey) *

13. Common Table Expressions
CTE
Create a set on the fly
Can only be used in a single query
Doesn’t actually create the set
Think of it like an include
Optimizer doesn’t see a separate set
--Common Table Expressions
with northAmerica as
(select [SalesTerritoryKey], [SalesTerritoryRegion]
from DimSalesTerritory
where [SalesTerritoryGroup] = 'North America')
select top 3 na.SalesTerritoryRegion, fis.SalesAmount
from FactInternetSales fis
inner join northAmerica na
on na.SalesTerritoryKey = fis.SalesTerritoryKey *

14. Union * Combine 2 sets with matching fields Useful in replacing Case
Field count must match exactly
Data types must be implicitly convertible
First query dictates names
Useful in replacing Case
Implicitly removes duplicates
Use ‘Union All’ to preserve duplicates
--Union
select distinct st.SalesTerritoryRegion
from FactInternetSales fis
inner join DimSalesTerritory st
on fis.SalesTerritoryKey = st.SalesTerritoryKey
union
from FactResellerSales frs
on frs.SalesTerritoryKey = st.SalesTerritoryKey *

15. Correlated Subquery * A necessary evil Alternatives
Appears in select list
Subquery contains a join referencing the outer query
Runs once for each record!
Alternatives
Join
Creating a temp record set
--Correlated Subquery
select e.lastName
,(select max(SalesAmount) from FactResellerSales frs where frs.EmployeeKey = e.EmployeeKey) as maxSales
from DimEmployee e
, max(frs.salesAmount)
inner join FactResellerSales frs
on frs.EmployeeKey = e.EmployeeKey
group by e.LastName *

16. Nulls What size dress does George wear?
Missing or inapplicable data
Nulls in any relational operation result in null
Nulls in string concatenation result in null
When testing for True, nulls appear false
Special functions
Is null (in WHERE clause)
Isnull()
Coalesce()

17. Cursors * Procedural Slow More code than SQL Can be the only solution
--Seperate products into free, cheap and expensive
select top 10 * from Production.Product
--cursor version
nvarchar(50)
money
char(10)
if object_id('tempdb..#priceGroups') is not null
drop table #priceGroups
create table #priceGroups (prod nvarchar(50), priceGroup char(10))
declare prices cursor for
select p.Name, p.ListPrice
from [Production].Product p
open prices
fetch next from
while = 0
begin
= 0
insert into #priceGroups values 'free')
> 0 < 100
insert into #priceGroups values 'cheap')
>= 100
insert into #priceGroups values 'expensive')
end
close prices
deallocate prices
select * from #priceGroups
--case version
select Name,
(case
when listPrice = 0 then 'free'
when listPrice > 0 and listPrice < 100 then 'cheap'
when listPrice >= 100 then 'expensive'
else 'priceless'
end) as priceGroup
from Production.Product
--union version
select Name, 'free' as priceGroup from Production.Product where ListPrice = 0
union
select Name, 'cheap' as priceGroup from Production.Product where ListPrice between 0 and 100
select Name, 'expensive' as priceGroup from Production.Product where ListPrice > 100 *

18. Recursive SQL * Use a CTE Create an anchor condition Union All
Query again calling anchor
Choose what you want outside the CTE
--Recursive CTE
with r as
(select c.AccountKey as ChildAccountKey, c.ParentAccountKey, 0 as lvl
from DimAccount c
where c.ParentAccountKey is null
union all
select p.AccountKey, p.ParentAccountKey, lvl + 1
from DimAccount p
inner join r
on p.ParentAccountKey = r.ChildAccountKey )
select parent.AccountDescription as parent, child.AccountDescription as child, lvl
from r
inner join DimAccount child
on r.ChildAccountKey = child.AccountKey
inner join DimAccount parent
on r.ParentAccountKey = parent.AccountKey
order by lvl, parent *

19. It’s important to understand data!
Query system tables
Profile data
--Profile
varchar(700)
varchar(50)
varchar(50)
varchar(100)
= ''
= ''
varchar(200)
DROP TABLE [dbo].[dbProfile]
IF EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[dbProfile]') AND type in (N'U'))
--Recreate the profile table
CREATE TABLE [dbo].[dbProfile](
[recordCnt] [int] NULL,
[ColumnName] [varchar](50) NOT NULL,
[TableName] [varchar](50) NOT NULL,
[distinctCnt] [int] NULL,
[fldSample] varchar(300)
[NullCnt] [int] NULL,
[nonNullCnt] [int] NULL,
) ON [PRIMARY]
SELECT t.name AS table_name,
declare dbSchema cursor for
c.name AS column_name
ORDER BY table_name
INNER JOIN sys.columns c ON t.OBJECT_ID = c.OBJECT_ID
FROM sys.tables AS t
while = 0
fetch next from
open dbSchema
begin
EXEC
= 'declare fldList cursor for select distinct ' + ' from '
open fldList
fetch next from fldList
while = 0 and < 200
+ + ', '
end
deallocate fldList
close fldList
= 'insert into dbProfile (TableName, ColumnName, recordCnt, distinctCnt, nonNullCnt, nullCnt, fldSample) '
+'count(distinct as distinctCnt, '
+'count(*) as recordCnt, '
+'select as TableName as ColumnName, '
+ '''' + + ''''
+'sum(case when 0 then 1 else 0 end) as NullCnt, '
+'sum(case when 0 then 0 else 1 end) as nonNullCnt, '
+' from '
begin try
end try
end catch
begin catch
deallocate dbSchema
close dbSchema
select * from dbProfile *

20. There’s a particular style to it…
Thinking in SQL
There’s a particular style to it…
Rob Hatton