sql basics joins

7/31/2019 SQL Basics Joins

1/35

SQL II


2/35

SQL Queries (so far)

Basic SQL Queries

SELECT-FROM-WHERE

Expressions and strings in SELECT and WHERE

UNION-type Queries Union, intersect, except queries

They do duplicate elimination by default!

You need to use ALL, eg. UNION ALL for retaining the duplicates.

Nested Queries

Allow for very complex queries


3/35

sid sname rating age

22 Dustin 7 45.0

31 Lubber 8 55.595 Bob 3 63.5

bid bname color

101 Interlake blue102 Interlake red

103 Clipper green

104 Marine red

sid bid day

22 101 10/10/96

95 103 11/12/96

Reserves

Sailors

Boats

We will use theseinstances ofrelations in our

examples.


4/35

Example Schemas


5/35

INTERSECT

SELECT S.sidFROM Sailors S, Boats B,

Reserves RWHERE S.sid=R.sid

AND R.bid=B.bidAND B.color=redINTERSECTSELECT S.sidFROM Sailors S, Boats B,


AND R.bid=B.bidAND B.color=green

Key field!

What if we used snameinstead of sid?


6/35

Find sids of sailors whove reserved a red but did not reserve agreen boat

SELECT S.sidFROM Sailors S, Boats B,


AND R.bid=B.bidAND B.color=redEXCEPTSELECT S.sidFROM Sailors S, Boats B,


AND R.bid=B.bidAND B.color=green

Duplicate eliminationIs important here!


7/35

Powerful feature of SQL: WHERE clause can itself contain an

SQL query!

Actually, so can FROM and HAVING clauses.

To find sailors whove notreserved #103, use NOT IN.

To understand semantics of nested queries:

think of a nested loopsevaluation: For each Sailors tuple, check thequalification by computing the subquery.

Nested Queries

SELECT S.sname

FROM Sailors SWHERE S.sid IN (SELECT R.sid

FROM Reserves RWHERE R.bid=103)

Names of sailors whove reserved boat #103:


8/35

More on Set-Comparison Operators

Weve already seen IN, EXISTS and UNIQUE. Can also use NOT

IN, NOT EXISTS and NOT UNIQUE.

Also available: opANY, opALL

Find sailors whose rating is greater than that of some sailor

called Horatio:

SELECT *FROM Sailors S

WHERE S.rating > ANY (SELECT S2.ratingFROM Sailors S2WHERE S2.sname=Horatio)


9/35

Basic SQL Queries - Summary

An advantage of the relational model is its

well-defined query semantics.

SQL provides functionality close to that ofthe basic relational model.

some differences in duplicate handling, null values, set operators,etc.

Typically, many ways to write a query

the system is responsible for figuring a fast way to actually

execute a query regardless of how it is written.


10/35

Aggregate Operators

COUNT (*)COUNT ( [DISTINCT] A)SUM ( [DISTINCT] A)AVG ( [DISTINCT] A)MAX (A)MIN (A)

SELECT AVG ( DISTINCT S.age)FROM Sailors S

WHERE S.rating=10

SELECT S.snameFROM Sailors SWHERE S.rating= (SELECT MAX(S2.rating)

FROM Sailors S2)

single column


11/35

Find name and age of the oldest sailor(s)

The first query is illegal in

SQL!

SELECT S.sname, MAX (S.age)FROM Sailors S

SELECT S.sname, S.ageFROM Sailors SWHERE S.age =

(SELECT MAX (S2.age)FROM Sailors S2)

SELECT S.sname, S.ageFROM Sailors SWHERE (SELECT MAX (S2.age)

FROM Sailors S2)

= S.age

Third query equivalent tosecond query allowed inSQL/92 standard, but notsupported in some systems.


12/35

Queries With GROUP BY

Thetarget-listcontains (i) list of column names &

(ii)terms with aggregate operations (e.g., MIN (S.age)).

column name list (i)can contain only attributes from thegrouping-list.

SELECT [DISTINCT] target-list

FROM relation-list[WHERE qualification]GROUP BYgrouping-list

Therefore we use a new operator in SQL, theGROUP BY clause


13/35

Group By Examples

SELECT S.rating, AVG (S.age)FROM Sailors SGROUP BY S.rating

For each rating, find the average age of the sailors

For each rating find the age of the youngestsailor with age 18

SELECT S.rating, MIN (S.age)FROM Sailors SWHERE S.age >= 18GROUP BY S.rating


14/35

Conceptual Evaluation

The cross-product ofrelation-listis computed, tuples that failqualificationare discarded, `unnecessary fields are deleted,and the remaining tuples are partitioned into groups by the

value of attributes in grouping-list.

One answer tuple is generated per qualifying group.


15/35

SELECT S.rating, MIN (S.age)FROM Sailors SWHERE S.age >= 18

GROUP BY S.rating


22 dustin 7 45.0

31 lubber 8 55.5

71 zorba 10 16.064 horatio 7 35.0

29 brutus 1 33.0

58 rusty 10 35.0

1. Form cross product

rating age

1 33.0

7 45.07 35.0

8 55.5

10 35.0

2. Delete unneeded columns,rows; form groups

3. PerformAggregation

ating age

1 33.0

7 35.08 55.0

10 35.0

Answer Table


16/35

Find the number of reservations foreach red boat.

Grouping over a join of two relations.

SELECT B.bid, COUNT(*)AS numresFROM Boats B, Reserves R

WHERE R.bid=B.bidAND B.color=red

GROUP BY B.bid


17/35

SELECT B.bid, COUNT (*) AS scountFROM Boats B, Reserves RWHERE R.bid=B.bid ANDB.color=redGROUP BY B.bid

1

b.bid b.color r.bid

101 blue 101

102 red 101

103 green 101104 red 101

101 blue 102

102 red 102

103 green 102

104 red 102

b.bid b.color r.bid

102 red 102

2

b.bid scount

102 1answer


18/35

Queries With GROUP BY and HAVING

Use the HAVING clause with the GROUP BY clause to

restrict which group-rows are returned in the result set

SELECT [DISTINCT] target-listFROM relation-listWHERE qualification

GROUP BY grouping-listHAVING group-qualification


19/35

Conceptual Evaluation

Form groups as before.

The group-qualificationis then applied toeliminate some groups.

Expressions in group-qualificationmust have a single value per group!

That is, attributes in group-qualificationmust be arguments of anaggregate op or must also appear in the grouping-list. (SQL does notexploit primary key semantics here!)

One answer tuple is generated per qualifyinggroup.


20/35

Find the age of the youngest sailor with age 18,for each rating with at least 2 such sailors

SELECT S.rating, MIN (S.age)FROM Sailors SWHERE S.age >= 18GROUP BY S.rating

HAVING COUNT (*) > 1

rating

7 35.0

Answer relation

rating age

1 33.0

7 45.0

7 35.0

8 55.5

10 35.0

2

rating m-age count

1 33.0 17 35.0 2

8 55.0 1

10 35.0 13

sid sname rating age22 Dustin 7 45.0

31 lubber 8 55.5

71 zorba 10 16.0

64 horatio 7 35.029 brutus 1 33.0

58 rusty 10 35.0


21/35

SELECT S.snameFROM Sailors SWHERE NOT EXISTS (SELECT B.bid

FROM Boats B

WHERE NOT EXISTS (SELECT R.bidFROM Reserves RWHERE R.bid=B.bid

AND R.sid=S.sid))

Sailors S such that ...

there is no boat B without...

a Reserves tuple showing S reserved B

Find sailors whove reserved all boats.


22/35

Can you do this using Group By and

Having?

SELECT S.name

FROM Sailors S, reserves RWHERE S.sid = R.sidGROUP BY S.name, S.sidHAVING

COUNT(DISTINCT R.bid) =( Select COUNT (*) FROM Boats)

Find sailors whove reserved all boats.

Note: must have both sid and name in the GROUP BY

clause. Why?


23/35

SELECT S.name, S.sid

FROM Sailors S, reserves R

WHERE S.sid = r.sidGROUP BY S.name, S.sid

HAVING

COUNT(DISTINCT R.bid) =

Select COUNT (*) FROM Boats

s.name s.sid r.sid r.bid

Dustin 22 22 101

Lubber 31 22 101

Bob 95 22 101Dustin 22 95 102

Lubber 31 95 102

Bob 95 95 102

s.name s.sid bcount

Dustin 22 1

Bob 95 1

bid bname color

101 Interlake blue

102 Interlake red103 Clipper green

104 Marine red

Count (*) from boats = 4

Apply having clause to groups

s.name s.sid


24/35

Null Values

Field values in a tuple are sometimes unknown(e.g., a ratinghas not been assigned) orinapplicable(e.g., no spousesname).

SQL provides a special value nullfor such situations.

The presence ofnullcomplicates many issues. E.g.: Special operators needed to check if value is/is not null.

Is rating>8true or false when ratingis equal to null? What aboutAND, OR andNOT connectives?

We need a 3-valued logic (true, false and unknown).

Meaning of constructs must be defined carefully. (e.g., WHERE clauseeliminates rows that dont evaluate to true.)

New operators (in particular, outer joins) possible/needed.


25/35

Joins

Explicit join semantics needed unless it is an INNER join(INNER is default)

SELECT (column_list)FROM table_name

[INNER | {LEFT |RIGHT | FULL } OUTER] JOIN table_nameON qualification_list

WHERE


26/35

Inner Join

Only the rows that match the search conditions are returned.

SELECT s.sid, s.name, r.bid

FROM Sailors s INNER JOIN Reserves r

ON s.sid = r.sidReturns only those sailors who have reserved boats

SQL-92 also allows:


FROM Sailors s NATURAL JOIN Reserves r

NATURAL means equi-join for each pair of attributes with thesame name


27/35

SELECT s.sid, s.name, r.bidFROM Sailors s INNER JOIN Reserves r

ON s.sid = r.sid

s.sid s.name r.bid22 Dustin 101

95 Bob 103


22 Dustin 7 45.0

31 Lubber 8 55.595 Bob 3 63.5

sid bid day

22 101 10/10/96

95 103 11/12/96


28/35

Left Outer Join

Left Outer Join returns all matched rows, plus all

unmatched rows from the table on the left of the join

clause

(use nulls in fields of non-matching tuples)


FROM Sailors s LEFT OUTER JOIN Reserves r

ON s.sid = r.sid

Returns all sailors & information on whether they have

reserved boats


29/35

SELECT s.sid, s.name, r.bidFROM Sailors s LEFT OUTER JOIN Reserves r

ON s.sid = r.sid

s.sid s.name r.bid22 Dustin 101

95 Bob 103

31 Lubber NULL


22 Dustin 7 45.0

31 Lubber 8 55.595 Bob 3 63.5

sid bid day

22 101 10/10/96

95 103 11/12/96


30/35

Right Outer Join

Right Outer Join returns all matched rows, plus all

unmatched rows from the table on the right of the join

clause

SELECT r.sid, b.bid, b.name

FROM Reserves r RIGHT OUTER JOIN Boats b

ON r.bid = b.bid

Returns all boats & information on which ones are

reserved.


31/35

SELECT r.sid, b.bid, b.nameFROM Reserves r RIGHT OUTER JOIN Boats b

ON r.bid = b.bid

r.sid b.bid b.name

22 101 Interlake102 Interlake

95 103 Clipper

NULL 104 Marine

sid bid day

22 101 10/10/96

95 103 11/12/96

bid bname color

101 Interlake blue

102 Interlake red

103 Clipper green104 Marine red


32/35

Full Outer Join

Full Outer Join returns all (matched or unmatched) rows

from the tables on both sides of the join clause

SELECT r.sid, b.bid, b.nameFROM Reserves r FULL OUTER JOIN Boats b

ON r.bid = b.bid

Returns all boats & all information on reservations


33/35

SELECT r.sid, b.bid, b.name

FROM Reserves r FULL OUTER JOIN Boats bON r.bid = b.bid

r.sid b.bid b.name

22 101 Interlake

102 Interlake

95 103 Clipper 104 Marine

Note: in this case it is the same as the ROJ becausebid is a foreign key in reserves, so all reservations must

have a corresponding tuple in boats.

sid bid day22 101 10/10/96

95 103 11/12/96

bid bname color

101 Interlake blue102 Interlake red

103 Clipper green

104 Marine red


34/35

Views

CREATE VIEW view_nameAS select_statement

Makes development simplerOften used for securityNot instantiated - makes updates tricky

CREATE VIEW Reds

AS SELECT B.bid, COUNT (*) AS scountFROM Boats B, Reserves RWHERE R.bid=B.bid AND B.color=redGROUP BY B.bid


35/35

CREATE VIEW Reds

AS SELECT B.bid, COUNT (*) AS scountFROM Boats B, Reserves R

WHERE R.bid=B.bid AND B.color=redGROUP BY B.bid

b.bid scount102 1

Reds

sql basics joins

Documents