1. CS122 Using Relational Databases and SQL
5/9/2018
CS122 Using Relational Databases and SQL
7. Advanced queries
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline Queries with UNION Sub-Queries in the FROM clause
Sub-Queries in the SELECT clause
Correlated Sub-Queries
EXISTS with Sub-Queries
Nested Sub-Queries
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3. Joins vs. Unions Table 1 Table 1 Table 2 Table 2 Union Join
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4. Queries UNION Combine data from different tables
ORDER BY clause in a UNION
UNION ALL
Using UNION to report both Detail and Total
Using UNION to report multiple statistics
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5. Combine data from different tables
Lyric Music wants to send Christmas cards to all members and all studios. Produce a Christmas card list containing the name and postal address of all members and studios
SELECT firstName, lastName, address, city, region, postalCode
FROM Members
UNION
SELECT studioName, ' ', address, city, region, postalCode
FROM Studios
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6. Union Rules Must have same number of fields
Use dummy fields if needed
Must have same data types in corresponding fields
Corresponding fields do not need to have the same names
Field names from first query will be used in results
Each query of the union is independent
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7. Another example List all studios and members who are the responsible
party for an artist, identifying the source as S or M
SELECT 'M' As Src, firstName, lastName, address, city,
region, postalCode
FROM Members M INNER JOIN XrefArtistsMembers X
ON M.MemberID = X.MemberID
WHERE respParty=1
UNION
SELECT 'S', studioName, ' ', address, city,region, postalCode
FROM Studios;
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8. ORDER BY clause in a UNION
A UNION can have only one
ORDER BY clause
It must be placed at the end of the query
It must refer to column names in the first SELECT clause
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9. Example Is this correct? SELECT 'M', firstName, lastName FROM Members
ORDER BY firstName
UNION ALL
SELECT 'S', studioName, ' '
FROM Studios
ORDER BY studioName;
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10. Example The correct one: SELECT 'M', firstName, lastName FROM Members
UNION
SELECT 'S', studioName, ' '
FROM Studios
ORDER BY firstName
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11. Union vs. Union All UNION eliminate duplicates
UNION ALL keeps duplicates
Example
SELECT region FROM Artists
UNION
SELECT region FROM Studios
ORDER BY region
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12. Using Union to Report Both Detail and Total
Union can also report the data from a single table massaged in different ways
Use Union to join the detail from a table with the aggregate from the same table to report both detail and totals together
Ex. Report all tracks and the total time of these tracks
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13. Using Union to Report Both Detail and Total
Select TrackNum, TrackTitle, LengthSeconds From Tracks
Union
Select 99, 'Total', Sum(LengthSeconds) From Tracks
Order by TrackNum
TrackNum TrackTitle LengthSeconds
Bob's Dream
My Wizard
Third's Folly
Leather
Hot Cars Cool Nights
Music in You
Don't Care About Time
Kiss
Pizza Box
Goodbye
Total
99 selected to fill column and allow proper sorting
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14. Using Union to Report Multiple Summary Statistics
Use Union to report multiple independent aggregates of some data and report all the results together
Ex. Report a count of tracks that are less than two minutes, between two and three minutes, and more than three minutes.
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15. Using Union to Report Multiple Summary Statistics
Select 1, '< 2 minutes' As Length, Count(*) As NumTracks
From Tracks Where LengthSeconds<120
Union
Select 2, '2-3 minutes', Count(*)
From Tracks Where LengthSeconds Between 120 and 180
Select 3, '>3 minutes', Count(*)
From Tracks Where LengthSeconds>180
Length NumTracks
< 2 minutes 21
minutes 22
>3 minutes 7
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16. Cautions on Union Overuse
Overuse can indicate poorly-designed data
Unions generally run much slower than Joins
For Joins the database can generally take advantage of indices on primary and foreign keys
But a Union (especially without All option) has to compare all the columns from every SELECT
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17. Outline Queries with UNION Sub-Queries in the FROM clause
Sub-Queries in the SELECT clause
Correlated Sub-Queries
EXISTS with Sub-Queries
Nested Sub-Queries
7. Advanced subqueries CS122_W16

18. Sub-Queries in FROM Syntax: Select fields
From Tabel1 Inner|Left|Right JOIN
(Select fields
From Table2) Alias
ON Table1.field=Alias.field
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19. Sub-Queries in FROM List all artists with members in Georgia.
Select Distinct Artistname
From (Artists A Inner Join XrefArtistsMembers X
On A.ArtistID = X.ArtistID) Inner Join
(Select MemberID
From Members M
Where M.Region='GA') M
On M.MemberID = X.MemberID
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20. Sub-Queries in FROM (Cont.)
5/9/2018
Sub-Queries in FROM (Cont.)
Another way:
Select Distinct Artistname
From Artists A Inner Join
(Select ArtistID From Members M
Inner Join XrefArtistsMembers X
On M.MemberID = X.MemberID
Where M.Region='GA') SC
On A.ArtistID = SC.ArtistID
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21. Outline Queries with UNION Sub-Queries in the FROM clause
Sub-Queries in the SELECT clause
Correlated Sub-Queries
EXISTS with Sub-Queries
Nested Sub-Queries
7. Advanced subqueries CS122_W16

22. Sub-Queries in Select Clause (non-correlated)
Syntax
SELECT Field, Field, (Select Field
From Table2)
FROM Table1
The sub-query MUST return
Only one row and one column
The sub-query can run independently
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23. Sub-Queries in Select Clause (non-correlated)
List track title, length in seconds, and the total seconds for the entire CD for all tracks in TitleID 4
Select titleid, TrackTitle, LengthSeconds
From Tracks
Where titleid=4
union
Select 4, 'Total', Sum(lengthseconds)
From Tracks
where titleid=4
How to make the total time appear in each row?
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24. Sub-Queries in Select Clause (non-correlated)
Use a subquery in Select clause!
Select titleid, TrackTitle, lengthSeconds, (Select Sum(lengthseconds)
From Tracks where titleid=4) As TotSec
From Tracks
Where titleid=4
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25. Sub-Queries in Select Clause (non-correlated)
List track title, length in seconds, and the total seconds and the length as a percentage of the total time for the CD for all tracks in TitleID 4
Select titleid, TrackTitle, LengthSeconds, (Select Sum(lengthseconds)
From Tracks where titleid=4) As TotSec,
LengthSeconds /(Select Sum(lengthseconds)
From Tracks where titleid=4) *100 As Percentage
From Tracks
Where titleid=4
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26. Outline Queries with UNION Sub-Queries in the FROM clause
Sub-Queries in the SELECT clause
Correlated Sub-Queries
EXISTS with Sub-Queries
Nested Sub-Queries
7. Advanced subqueries CS122_W16

27. Correlated Sub-Queries
Syntax
SELECT Field, Field, (Select Field
From Table2
Where table2.Field=Table1.Field)
FROM Table1
WHERE clause in sub-query joins each row to appropriate row of the outer query
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28. Example
List track title, length in seconds, and the total seconds for the entire CD for all tracks in TitleID 4 in each titleID
SELECT titleid,trackTitle, lengthSeconds As Sec,
( Select Sum(lengthseconds)
From Tracks SC
Where SC.titleID=T.titleID) As TotSec
FROM Tracks T
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29. Outer Table: Tracks (T)
TitleID
TrackTitle
Length 1
Song 1 10
Song 2 30 2
Song 3 20
Song 4
Song 5 40
Sub-query
Select Sum(lengthseconds)
From Tracks SC
Where SC.titleID=T.TitleID
Query results 1
TitleID
TrackTitle
Length
TotalSec 1
Song 1 10 40
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30. Outer Table: Tracks (T)
TitleID
TrackTitle
Length 1
Song 1 10
Song 2 30 2
Song 3 20
Song 4
Song 5 40
Sub-query
Select Sum(lengthseconds)
From Tracks SC
Where SC.titleID=T.TitleID
Query results 1
TitleID
TrackTitle
Length
TotalSec 1
Song 1 10 40
Song 2 30
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31. Outer Table: Tracks (T)
TitleID
TrackTitle
Length 1
Song 1 10
Song 2 30 2
Song 3 20
Song 4
Song 5 40
Sub-query
Select Sum(lengthseconds)
From Tracks SC
Where SC.titleID=T.TitelID
Query results 2
TitleID
TrackTitle
Length
TotalSec 1
Song 1 10 40
Song 2 30 2
Song 3 20 90
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32. Outer Table: Tracks (T)
TitleID
TrackTitle
Length 1
Song 1 10
Song 2 30 2
Song 3 20
Song 4
Song 5 40
Sub-query
Select Sum(lengthseconds)
From Tracks SC
Where SC.titleID=T.TitelID
Query results 2
TitleID
TrackTitle
Length
TotalSec 1
Song 1 10 40
Song 2 30 2
Song 3 20 90
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33. Outer Table: Tracks (T)
TitleID
TrackTitle
Length 1
Song 1 10
Song 2 30 2
Song 3 20
Song 4
Song 5 40
Sub-query
Select Sum(lengthseconds)
From Tracks SC
Where SC.titleID=T.TitelID
Query results 2
TitleID
TrackTitle
Length
TotalSec 1
Song 1 10 40
Song 2 30 2
Song 3 20 90
Song 5
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34. Correlated Sub-Queries
Sub-query returns multiple rows without a GROUP BY and a non-aggregate in SELECT
Refers in the sub-query to T table alias that exists only in the outer query
 Because of unique syntax the sub-query cannot be debugged as independent query
Select TrackTitle, LengthSeconds As Sec,
(Select Sum(lengthseconds) From Tracks SC
Where SC.TitleID=T.TitleID) As TotSec
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35. The same example Use From clause subquery to answer the same question:
SELECT t.titleid,tracktitle, lengthseconds, totsec
FROM Tracks t inner join
(select titleid,
sum(lengthseconds) as totsec
from Tracks
group by titleid) sc
on sc.titleid=t.titleid
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36. Outline Queries with UNION Sub-Queries in the FROM clause
Sub-Queries in the SELECT clause
Correlated Sub-Queries
EXISTS with Sub-Queries
Nested Sub-Queries
7. Advanced subqueries CS122_W16

37. EXISTS with Sub-Queries
Syntax
SELECT Field1, Field2
FROM Table1
Where Exists (Select Field1, Field2
from Table2
Where Table1.Field1=Table2.Field1)
Sub-query returns one or more rows which are correlated with rows of outer query
If any matching sub-query rows are found, row from outer query is reported
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38. EXISTS with Sub-Queries
EXISTS checks for the existence of data in the sub-query
Data is either there (True) or it isn't (False)
Often used as a WHERE clause sub-query but with correlated sub-query syntax
Example:
Report a list of all artists with more than one member
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39. EXISTS with Sub-Queries
Select Artistname From Artists A
Where Exists
(Select ArtistID, Count(MemberID)
From XrefArtistsMembers X
Where X.ArtistID= A.ArtistID
Group by ArtistID
Having Count(MemberID)>1)
Artistname
Word
Sonata
The Bullets
Confused
The Kicks
Today
21 West Elm
Highlander
ArtistID
Sub-query results
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40. Correlated Sub-Queries vs. Joins
Correlated sub-query
Select Artistname From Artists A
Where Exists (Select ArtistID, Count(MemberID) From XrefArtistsMembers X Where X.ArtistID= A.ArtistID Group by ArtistID Having Count(MemberID)>1)
Select Artistname From Artists A
Inner Join
(Select ArtistID, Count(MemberID) as NumMem
From XrefArtistsMembers Group by ArtistID
Having Count(MemberID)>1) SQ
On A.ArtistID = SQ.ArtistID
Join with same results
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41. Outline Queries with UNION Sub-Queries in the FROM clause
Sub-Queries in the SELECT clause
Correlated Sub-Queries
EXISTS with Sub-Queries
Nested Sub-Queries
7. Advanced subqueries CS122_W16

42. Nested Sub-queries SQL allows nesting sub-queries within sub-queries
Number of levels allowed is not so important
You will get confused before SQL does
Example
List the names of all the artists who have recorded a title with ten or more tracks
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43. Example Select Artistname From Artists A Where Exists
(Select ArtistID From Titles T
Where T.ArtistID= A.ArtistID
And Exists
(Select TitleID, Count(TrackNum)
From Tracks TR
Where TR.TitleID = T.TitleID
Group by TitleID
Having Count(Tracknum)>=10) )
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