1. NURS6803 Clinical DB Design Katherine Sward, PhD, RN
More SQL
NURS6803
Clinical DB Design
Katherine Sward, PhD, RN 1

2. Planning the query (the process I use)
Write the template Select From Where
Determine which table(s) have the data I need
What columns do I want to return
What do I need in order to determine what rows I want

3. Planning the query If it’s complex, I do just a portion at a time
Once that’s working the way I want, then do the next part
You’ll see this in action as we work through examples

4. Subqueries
In relational theory… the result of manipulating a relation (or multiple relations) is also a relation
We think of relation as a table
But the result of a query is also a “relation”
So we can use the result of a query like it’s a table (temporary table – does not copy data to a new physical table)
The result of one query can be the “source” for another query

5. View
When you save the definition of a query (save the query, in Access) you create a VIEW.
A view is simply a saved query definition
You do not have to save the query as a view in order to base another query on it’s results

6. Subqueries
You can embed a complete SELECT statement (complete query) within another query
This is called a nested query (or subquery)
Subquery returns
A single value
A single row of data with multiple columns
Multiple rows of data with one or more columns

7. Subqueries
Suppose all the columns you want to display are in one table
But the criteria to determine what ROWS to display are in another table
Let’s say we want patient ID and name for everyone who is the same race as Deanna Troi
This is all in the demog table

8. Build the query Names are formatted Last, First
So first find Deanna’s race
SELECT race
FROM demog
WHERE name = 'Troi, Deanna'
NOTE: straight quotes, not curvy ones!
YES '
NO ‘ or “
MAYBE "

9. Build the query
Now find the people who have the same value as Deanna for race
SELECT pt_id, name
FROM demog
WHERE race =
(SELECT race
WHERE name = 'Troi, Deanna')

10. Subqueries Now, suppose the criterion is in a different table
Let’s say we want patient ID and name for every patient who has a low WBC (less than 3.8). We don’t want the wbc value, just the ID and name.

11. What tables?

12. What tables I only want to return columns from demog
But the rows will be determined by data in the vitals table
The things we want to return are the outer query
The filter is the subquery
So we will use vitals for the subquery

13. WHERE WBC < 3.8; returns a list of pt_id
SELECT pt_id
FROM blood
WHERE WBC < 3.8; returns a list of pt_id
pt_id
103
133 15
155

14. Now move up a layer
Put parentheses around the subquery and get rid of the semi-colon
(SELECT pt_id
FROM blood
WHERE WBC < 3.8)

15. IN
We will use the keyword IN to tell the database that we are providing a list
You saw IN with a list previously when we made a list of zip codes
Our query returned a list, so we can use the query in place of typing out the results

16. The whole query SELECT pt_id, name FROM demog WHERE pt_id IN
FROM blood
WHERE WBC < 3.8);
pt_id
name
103
Cole, Nat-King
133
Newman, Paul 15
Drexel, Clyde
155
Robert, Jake 2
Minamino, Yoko
213
Troi, Deanna
214
Devil, Tazmanian
247
null, null
314
378
396
454
485

17. The whole query SELECT pt_id, name FROM demog WHERE pt_id IN
FROM blood
WHERE WBC < 3.8)
ORDER BY cint(pt_id);
pt_id
name 2
Minamino, Yoko 15
Drexel, Clyde
103
Cole, Nat-King
133
Newman, Paul
155
Robert, Jake
213
Troi, Deanna
214
Devil, Tazmanian
247
null, null
314
378
396
454
485

18. Subqueries
Suppose your criteria for “what rows” includes an aggregate function or a list of items
You can use any legal comparator that makes sense for the query (less than, greater than, etc.)
But be careful. Know whether your subquery returns one row, or multiple

19. This time everything we want is in the vitals table
For example, we want to know the pt_id for everyone who has a SBP higher than the average SBP for the group
This time everything we want is in the vitals table
We will use a subquery to find the average SBP
Then use the outer query to find everyone higher than that

20. SELECT pt_id FROM vitals WHERE SBP > (SELECT avg(SBP) FROM vitals)
Note that the field in the outer table is to the LEFT of the > symbol, and the subquery is to the right
SELECT pt_id
FROM vitals
WHERE SBP >
(SELECT avg(SBP)
FROM vitals)

21. We can even do math with the subquery
We can even do math with the subquery. Suppose we wanted people who have an SBP higher than 1.5 times the average SBP for the group….

22. SELECT pt_id
FROM vitals
WHERE SBP > 1.5 *
(SELECT avg(SBP)
FROM vitals)

23. Other uses You can use a subquery in the SELECT clause
You can use a subquery in the FROM clause (this is a view when used in the FROM clause)

24. Getting tricky…
Suppose we want to know how each person’s highest SBP compares to the average for the group—we want to know how much above average is the highest SBP, for those folks who have higher-than-average SBP

25. Building a little at a time
Start with the query that finds the average SBP for the group SELECT avg(SBP) FROM vitals

26. Building a little at a time
Who has higher than average SBP? SELECT pt_id FROM vitals WHERE SBP > (SELECT avg(SBP) FROM vitals)

27. Building a little at a time
What is max SBP for people who have higher than average SBP? SELECT pt_id, max(SBP) as MaxBP FROM vitals WHERE SBP > (SELECT avg(SBP) FROM vitals) GROUP BY pt_id

28. Building a little at a time
What is difference between each individual’s max SBP and the overall average SBP? SELECT pt_id, max(SBP) as MaxBP, max(SBP) – (SELECT avg(SBP) from vitals) As BPDiff FROM vitals WHERE SBP > (SELECT avg(SBP) FROM vitals) GROUP BY pt_id ORDER BY cint(pt_id)
pt_id
MaxSBP
BPDiff 1
130 3
138 6
140 14
150 16 17

29. ANY and ALL
If the subquery returns a single column (can be multiple rows, but a single column)…
You can use ALL (only true if ALL values of the subquery are matched)
You can use ANY (true if at least one value in the subquery is matched). ANY is same as SOME.
If you do not specify, then “ANY” is used

30. Example
Suppose you have a table with letters of the alphabet (go with me here for the sake of syntax example…)
SELECT Letter From Alphabet where Letter > ALL (Select letter from alphabet Where letter < D)

31. Example SELECT Letter From Alphabet where Letter > ANY (A, B, C)
SELECT Letter From Alphabet where Letter > ALL (A, B, C)
Without ALL, it will return B, C, D, E….. (B > A, C>B and C>A)
With ALL then B and C are not in the results

32. Subquery is less commonly used than what we will do next – joining tables
When you want data from more than one table in your results….