1. CS4222 Principles of Database System
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CS4222 Principles of Database System
11. SQL
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline Basic queries (only one table) Joins Subqueries Set operations
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Outline
Basic queries (only one table)
Joins
Subqueries
Set operations
DML
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3. Retrieval Queries in SQL
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Retrieval Queries in SQL
SQL has one basic statement for retrieving information from a database; the SELECT statement
This is not the same as the SELECT operation of the relational algebra
Important distinction between SQL and the formal relational model:
SQL allows a table (relation) to have two or more tuples that are identical in all their attribute values
Hence, an SQL relation (table) is a multi-set (sometimes called a bag) of tuples; it is not a set of tuples
SQL relations can be constrained to be sets by specifying PRIMARY KEY or UNIQUE attributes, or by using the DISTINCT option in a query
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4. Retrieval Queries in SQL
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Retrieval Queries in SQL
A bag or multi-set is like a set, but an element may appear more than once.
Example: {A, B, C, A} is a bag. {A, B, C} is also a bag that also is a set.
Bags also resemble lists, but the order is irrelevant in a bag.
Example:
{A, B, A} = {B, A, A} as bags
However, [A, B, A] is not equal to [B, A, A] as lists
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5. Retrieval Queries: SELECT-FROM-WHERE block
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Retrieval Queries: SELECT-FROM-WHERE block
Basic form of the SQL SELECT statement is called a mapping or a SELECT-FROM-WHERE block
SELECT <attribute list>
FROM <table list>
WHERE <condition>
<attribute list> is a list of attribute names whose values are to be retrieved by the query
<table list> is a list of the relation names required to process the query
<condition> is a conditional (Boolean) expression that identifies the tuples to be retrieved by the query
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6. Example database Sailors Boats Reserves
(sid:integer, sname:string, rating:integer, age:real)
Boats
(bid:integer, bname:string, color:string)
Reserves
(sid:integer, bid:integer, day:date)
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7. Basic queries Basic format Distinct, Order by and Like
Aggregate functions
Group by and Having
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8. Basic format
SELECT att1, att2,…
FROM table1
WHERE conditions
Ex1: Find the names of the sailors whose rating is greater than 5
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9. Example Ex1: Find the names of the sailors
whose rating is greater than 5
SELECT sname
FROM Sailors
WHERE rating>5
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10. 1/11/2019
USE OF *
To retrieve all the attribute values of the selected tuples, an * is used, which stands for all the attributes
Examples:
SELECT *
FROM Boats
WHERE color = 'red'
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11. DISTINCT: remove duplicates
SQL does not treat a relation as a set; duplicate tuples can appear
To eliminate duplicate tuples in a query result, the keyword DISTINCT is used
Ex2: Find the names (without duplicates) of the sailors whose rating is greater than 5
SELECT DISTINCT sname
FROM Sailors
WHERE rating > 5
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12. Order by: sort the results
Ex3: Find the names and the ratings of the sailors whose rating is greater than 2. Order the results by the rating
SELECT sname, rating
FROM Sailors
WHERE rating>2
ORDER BY rating
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13. More on ORDER BY Not SORT BY The default is ASC
Use DESC if you want descending order
ORDER BY rating DESC
Order by multiple attributes
ORDER BY rating DESC, name DESC
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14. LIKE: Pattern matching
Wildcards
_: matches any single character
%: matches any string of zero or more characters
'abc' LIKE 'abc' true
'abc' LIKE 'a%' true
'abc' LIKE '_b_' true
'abc' LIKE 'c' false
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15. LIKE: example
Ex4: Find the boats with ‘un’ anywhere in the boat’s name
SELECT *
FROM Boats
WHERE bname LIKE '%un%'
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16. LIKE: example
Ex5: Find the boats with ‘i’ in the second left most character of the boat name
SELECT *
FROM Boats
WHERE bname LIKE '_i%'
Is the following query correct?
FROM
WHERE bname = '_i%'
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17. Aggregate functions Max() Min() Avg() Count() Sum()
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18. Aggregate functions: example
Ex6. Find the number of sailor’s , the maximum, minimum and average rating
SELECT Count(*), Max(rating), Min(rating), Avg(rating)
FROM Sailors
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19. Group By Is the following query correct?
Select rating, Count(*)
From Sailors
When attributes and aggregation functions are listed together in the Select clause, Group by must be used.
Group by rating
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20. Having: remove groups Always used with Group by
Ex7. For each rating, report the number of sailors. Only Keep the rating with the number of sailors greater than 1
Select rating, Count(*)
From Sailors
Group by rating
Having count(*) > 1
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21. Having: remove groups The differences between WHERE and HAVING
Where: remove tuples to be aggregated
Having: final filter, remove groups
Ex8. For each rating, report the number of sailors whose sname contains ‘s’. Only Keep the rating with the number of such sailors greater than 1
Select rating, Count(*)
From Sailors
Where sname LIKE ‘%s%’
Group by rating
Having count(*) > 1
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22. Joins Equi Join Inner Join Natural Join Outer join Left join
Right join
Produce the same results
The syntax is different
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23. Ambiguous attribute names
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Ambiguous attribute names
In SQL, we can use the same name for two (or more) attributes as long as the attributes are in different relations
A query that refers to two or more attributes with the same name must qualify the attribute name with the relation name by prefixing the relation name to the attribute name
Example: Sailors.sid, Reserves.sid
Can also use the AS keyword to specify aliases
AS can be omitted
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24. Equi Join
Ex9. Find the names of sailors who have reserved boat number 10
SELECT S.sname
FROM Sailors S, Reserves R
WHERE S.sid=R.sid And R.bid=10
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25. UNSPECIFIED WHERE-clause
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UNSPECIFIED WHERE-clause
A missing WHERE-clause indicates no condition; hence, all tuples of the relations in the FROM-clause are selected
This is equivalent to the condition WHERE TRUE
If more than one relation is specified in the FROM-clause and there is no join condition, then the CARTESIAN PRODUCT of tuples is selected
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26. UNSPECIFIED WHERE-clause
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UNSPECIFIED WHERE-clause
Example:
SELECT S.sname
FROM Sailors S, Resvers R
It is extremely important not to overlook specifying any selection and join conditions in the WHERE-clause; otherwise, incorrect and very large relations may result
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27. Inner join
Ex9. Find the names of sailors who have reserved boat number 10
SELECT S.sname
FROM Sailors S INNER JOIN Reserves R
ON S.sid=R.sid
WHERE R.bid=10
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28. Natural join
Ex9. Find the names of sailors who have reserved boat number 10
SELECT S.sname
FROM Sailors S NATURAL JOIN Reserves R
WHERE R.bid=10
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29. Outer join Left join Right join
Report all of the records of the first (left) of two tables plus matching records in the second (right) table.
Right join
Report all of the records of the second (right) of two tables plus matching records in the first (left) table
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30. Left Join example
Ex10. Report the names of all sailors and the boat ids (if any) they’ve reserved.
Select S.sname, R.bid
From Sailors S LEFT JOIN Reserves R
ON S.sid = R.sid
From Sailors S NATURAL LEFT JOIN Reserves R
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31. Inner Join vs. Out Join Select sname, bid Select sname, bid
sid
sname
rating
age 22
Dustin 7 45 29
Brutus 1 33 31
Lubber 8 55
sid
bid
day 22
101
1/1/98
102 31
103
1/8/98
sname
bid
Dustin
101
102
Brutus
NULL
Lubber
103
Select sname, bid
From Sailors NATURAL LEFT JOIN Boats
sname
bid
Dustin
101
102
Lubber
103
Select sname, bid
From Sailors NATURAL JOIN Boats
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32. Null Values Field values in a tuple are sometimes Test the NULL value
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Null Values
Field values in a tuple are sometimes
unknown (e.g., a rating has not been assigned)
inapplicable (e.g., no spouse’s name).
Test the NULL value
IS NULL
IS NOT NULL
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33. What is a subquery?
A complete SELECT query, called a subquery, can be specified within the WHERE-clause of another query, called the outer query
Where to put a subquery?
Where
Having
Select
From
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34. Subqueries in WHERE Ex.11 Find the name of the oldest sailor
Is this query correct?
Select sname, max(age)
From Sailors
Select sname
Where age = (Select max(age)
From Sailors)
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35. Another subquery example
In/NOT IN usually used with subqueries
Ex.9 Find the name of sailors who have reserved boat 10
Select sname
From Sailors S
Where S.sid IN (Select R.sid
From Reserves R
Where R.bid=10)
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36. Subqueries in Having
Ex.12 Find the rating with the maximum number of sailors
Select rating, count(*)
From Sailors S
Group by rating
Having count(*) >=ALL (Select count(*)
From Sailors
Group by rating)
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