Microsoft’s
Measured Skill description: This objective may include but is not
limited to: use the ranking function to select top(X) rows for multiple
categories in a single query; write and perform queries efficiently using the
new code items such as synonyms and joins (except, intersect); implement logic
which uses dynamic SQL and system metadata; write efficient, technically
complex SQL queries, including all types of joins versus the use of derived
tables; determine what code may or may not execute based on the tables
provided; given a table with constraints, determine which statement set would
load a table; use and understand different data access technologies; CASE
versus ISNULL versus COALESCE
What
I see:
·
ranking functions
·
except and intersect
·
data access technologies
·
CASE vs. ISNULL vs. COALESCE
Ranking Functions
There are four ranking functions
available in SQL Server: RANK, DENSE_RANK,
ROW_NUMBER, and NTILE.
As per BOL,
the RANK function “returns the rank of each row within the partition of a
result”:
use MeasureMySkills;
go
create table RankingTable
(
id int identity(1, 1) not null,
IntValue int not null
);
go
insert into RankingTable(IntValue)
values
(16),
(17),
(17),
(19),
(26),
(25),
(26),
(26),
(30);
select
id,
IntValue,
rank() over
(
order by IntValue
) as Rank
from RankingTable;
As you can
see from the above result, ties are the same rank but then subsequent rank
values reflect the gap caused by the ties.
If you want to fill those gaps (i.e. have a “dense” ranking), use the
DENSE_RANK function:
select
id,
IntValue,
dense_rank() over
(
order by IntValue
) as DenseRank
from RankingTable;
Another
common ranking function is ROW_NUMBER.
This ranking function will assign a sequential, non-duplicated “row
number” to the result set (very handy for deleting duplicate data through the
use of the windowing PARTITION BY clause):
select
id,
IntValue,
row_number() over
(
order by IntValue
) as RowNumber
from RankingTable;
And the last
ranking function is NTILE. This ranking
function breaks the windowed data into the specified number of groups:
select
id,
IntValue,
ntile(3) over
(
order by IntValue
) as NTile
from RankingTable;
EXCEPT and INTERSECT
These two operators return the
same distinct rows from two queries (INTERSECT) or missing distinct rows from
first query that aren’t in the second query (EXCEPT). Below shows examples of this output:
create table IntersectExceptTbl1
(
MyInt int not null,
MyString nvarchar(128) not null
);
go
create table IntersectExceptTbl2
(
MyInt int not null,
MyString nvarchar(128) not null
);
go
insert into IntersectExceptTbl1(MyInt, MyString)
values
(1,
'a'),
(2,
'b'),
(3,
'c'),
(4,
'd'),
(5,
'e');
insert into IntersectExceptTbl2(MyInt, MyString)
values
(1,
'a'),
(6,
'b'),
(3,
'c'),
(4,
'f'),
(5,
'e');
select *
from IntersectExceptTbl1
intersect
select *
from IntersectExceptTbl2;
/* RESULT SET (INTERSECT)
MyInt MyString
1 a
3 c
5 e */
select *
from IntersectExceptTbl1
except
select *
from IntersectExceptTbl2;
/* RESULT SET (EXCEPT)
MyInt MyString
2 b
4 d */
Data Access Technologies
For more information, please see
this MSDN reference on the
Data Access
Technologies Road Map.
It is worth
familiarizing yourself with the most commonly-used methods.
CASE vs. ISNULL vs. COALESCE
Choosing between CASE, ISNULL,
and COALESCE just comes down to what’s the best tool for the given task. CASE is typically used for relatively complex
conditional logic that COALESCE can’t encompass (for instance, not driven by
NULL). COALESCE is really just a
restricted and abbreviated CASE expression.
ISNULL is simply used to return either the first parameter’s value, or
if that’s NULL then it’ll return the second.
This is great for ensuring that NULL isn’t passed to aggregating
functions, or any other time that NULL is less than desired.
References
If there are any comments, questions, issues, or suggestions please feel
free to leave a comment below or email me at
sqlsalt@gmail.com.
