This document provides an overview of keys and joins in SQL. It discusses the different types of keys like primary keys, foreign keys, and unique keys. It also covers the different types of joins like inner joins, outer joins, and self joins. The document provides examples of creating keys and using different join types. It discusses some performance trade-offs between joins and alternatives like using multiple update statements instead of cursors.

1. INTERNAL TECHNICAL
TRAINING SESSION FOR
SQL
BY :- RAHUL KUMAR SINGH
(JAVA TEAM)

2. KEYS
 Key is a single or combination of multiple fields in a table. It is used to
fetch or retrieve records/data-rows from data table according to the
condition/requirement. Keys are also used to create relationship
among different database tables or views.
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3. TYPES OF KEYS IN SQL
 Candidate key
 Super key
 Primary key
 Alternative key
 Foreign key
 Unique key
 In our company database primary key and foreign keys are used
heavily
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4. CANDIDATE KEY
 Minimum set of attributes used to differentiate records of the table
 (sid,cid) candidate key
 Subset of (sid,cid) should not be a candidate key.
 If candidate key forms with single attribute then the candidate key is
called simple candidate key, otherwise it’s called compound candidate
key.
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Sid cid Fee
S1 C1 5000
S1 C2 6000
S2 C2 7000
S2 C3 7000

5. PRIMARY KEY
 One of the candidate key is primary key. Candidate key with not null
attribute is called primary key.
 cid is primary key here – all distinct values with not null constraint.
 cid is candidate key also(simple candidate key).
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Sid cid Fee
S1 C1 5000
S1 C2 6000
S2 C3 7000
S2 C4 8000

6. PRIMARY KEY(Contd.)
 A RDBMS can have only one primary key.
 No primary key in this table.
 Is this table valid for mysql InnoDB engine?
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Sid cid Fee
S1 C1 5000
S1 C2 6000
S2 C2 7000
S2 C3 7000

7. PRIMARY KEY(Contd.)
 In mysql, the InnoDB storage engine always creates a primary key, if
you didn’t specify it explicitly, thus making an extra column you don’t
have access to.
 Primary key can be composite.
 SYNTAX TO CREATE PRIMARY KEY
CREATE TABLE roll (
id int(11) NOT NULL AUTO_INCREMENT,
surname varchar(100) NOT NULL,
PRIMARY KEY(id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
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8. ALTERNATIVE KEY
 All candidates keys accept primary key is alternative key.
 Here sid, cid and fee are candidate keys. So if we are considering sid
as a primary key than cid and fee are alternative keys.
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Sid cid Fee
S1 C1 5000
S1 C2 6000
S2 C3 7000
S2 C4 8000

9. SUPER KEY
 Set of attributes used to differentiate records.
 Super keys are :- cid , fee , (cid,sid) , (cid,fee) , (sid,fee)
 Every candidate key is super key but every super key is not candidate
key.
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Sid cid Fee
S1 C1 5000
S1 C2 6000
S2 C3 7000
S2 C4 8000

10. UNIQUE KEY
 Set of one or more fields/columns of a table that uniquely identify a
record. It is like a primary key but it can accept only one null value
and it cannot have duplicate values.
 Can have more than one unique key constraint per table.
 One NULL is allowed because null is unique in itself, hence like similar
to primary key.
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11. UNIQUE KEY(Contd.)
 SYNTAX TO CREATE UNIQUE KEY
CREATE TABLE roll (
id int(11) NOT NULL AUTO_INCREMENT,
surname varchar(100) NOT NULL,
PRIMARY KEY(id),
UNIQUE KEY(roll)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
 Mysql InnoDB engine allows multiple null values for unique keys.
 Depending on the storage engine null may or may not be seen as a
unique value.
 BDB engine doesn’t allow multiple null whereas MYISAM & InnoDB
engine allows multiple nulls for unique key constraint.
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12. FOREIGN KEY
 A relation r1 may include among it’s attribute the primary key of an
other relation say r2, the attribute is called foreign key from r1
referencing r2.
r1 – referencing relation
r2 – referenced relation
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13. FOREIGN KEY(Contd.)
CREATE TABLE roll (
id int(11) NOT NULL AUTO_INCREMENT,
surname varchar(100) NOT NULL,
PRIMARY KEY(id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
CREATE TABLE student (
id int(11) NOT NULL AUTO_INCREMENT,
rollno int(11) NOT NULL,
name varchar(100) NOT NULL,
PRIMARY KEY(id),
FOREIGN KEY(rollno) REFERENCES roll(id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
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14. FOREIGN KEY(Contd.)
ID SURNAME
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ID rollno name
REFERENCING RELATION
REFERENCED RELATION

15. REFERENTIAL INTEGRITY CONSTRAINT
 ON DELETE NO ACTION
 ON DELETE CASCADE
 ON DELETE SET NULL
CREATE TABLE student (
id int(11) NOT NULL AUTO_INCREMENT,
rollno int(11) NOT NULL,
name varchar(100) NOT NULL,
PRIMARY KEY(id),
FOREIGN KEY(rollno) REFERENCES roll(id) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
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16. DROP FOREIGN KEY
SHOW CREATE TABLE student; //will give you foreign keys name
tablename_ibfk_[number] //shorthand naming convention for
InnoDB engine
ALTER TABLE student DROP FOREIGN KEY [foreign key name]
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17. SIMPLE SQL QUERY
SELECT table1.A, table1.B, table2.C
FROM table1, table2
WHERE table1.B = table2.B
GROUP BY(attribute)
HAVING(condition)
ORDER BY(attribute(desc))
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18. ORDER OF QUERY EXECUTION
6) SELECT table1.A, table1.B, table2.C
1) FROM table1, table2
2) WHERE table1.B = table2.B
3) GROUP BY(attribute)
4) HAVING(condition)
5) ORDER BY(attribute(desc))
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19. EXECUTION OF AN SQL QUERY
A B
1 2
3 4
5 6
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A B
2 3
4 5
9 10
A B B C
1 2 2 3
1 2 4 5
1 2 9 10
3 4 2 3
3 4 4 5
3 4 9 10
5 6 2 3
5 6 4 5
5 6 9 10
TABLE 1
TABLE 2
TABLE 1 X TABLE 2
CARTESIAN
PRODUCT

20. TRADE OFF BETWEEN TIME AND SPACE
 If a table has 5000000 entries and joins with another table with same
number of entries, then the joined table(table formed with cartesian
product) will contain 25 trillion entries.
 Query performances increases with multiple joins but it requires
more resources. Depending on the requirement and availability of
resources, one should how joins should be used.
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21. JOINS
 JOINS are used to combine rows from two or more tables to save
time(improves query performance).
 TYPES OF JOINS
 INNER JOIN
LEFT OUTER JOIN
 OUTER JOIN RIGHT OUTER JOIN
FULL OUTER JOIN
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22. JOINS(Contd.)
 JOINS is nothing but cartesian product of two or more tables
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A B C
1 2 3
3 1 2
A B C
2 3 4
2 5 1
A B C B C D
1 2 3 2 3 4
1 2 3 2 5 1
3 1 2 2 5 1
3 1 2 2 3 4
R : S :
R JOIN S :

23. JOINS(Contd.)
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24. JOINS(Contd.)
SELECT person.lastname, person.firstname, orders.orderno
FROM persons INNER JOIN orders
ON persons.pid = orders.pid
SELECT person.lastname, person.firstname, orders.orderno
FROM persons, orders
WHERE persons.pid = orders.pid
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25. JOINS(Contd.)
EXPLICIT JOIN
SELECT person.lastname, person.firstname, orders.orderno
FROM persons INNER JOIN orders
ON persons.pid = orders.pid
IMPLICIT JOIN (ANSI SQL 92 SYNTAX)
SELECT person.lastname, person.firstname, orders.orderno
FROM persons, orders
WHERE persons.pid = orders.pid
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26. JOINS(Contd.)
 NATURAL JOIN works same as JOIN but it eliminates duplicate
columns after matching
 SELECT * FROM student JOIN roll // simple cartesian product
 SELECT * FROM student NATURAL JOIN roll; // no duplicates and checks
for matching fields
 SELECT * FROM student, roll //duplicates and checks for
WHERE student.id = roll.id; matching fields
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27. JOINS(Contd.)
 SELECT * FROM student, roll; //cartesian product
 SELECT * FROM student JOIN roll; //cartesian product
 SELECT * FROM student JOIN roll //cartesian product, duplicates, with on
ON student.id = roll.id; condition only relevant data
 SELECT * FROM student INNER JOIN roll //cartesian product, duplicates,
ON student.id = roll.id; with on condition only relevant
data, without on condition
only cartesian product
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28. JOINS(Contd.)
 SELECT * FROM roll NATURAL LEFT OUTER JOIN student;
 SELECT * FROM roll NATURAL RIGHT OUTER JOIN student;
 In MYSQL FULL OUTER JOIN is implemented by UNION of LEFT OUTER JOIN
AND RIGHT OUTER JOIN
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29. JOINS – SELF JOIN
 The SQL SELF JOIN is used to join a table to itself as if the table were
two tables, temporarily renaming at least one table in the SQL
statement.
 Name of all employee that are from same location from where A
belong?
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NAME LOCATION
A New york
B Canada
C New york
Employee :

30. JOINS – SELF JOIN(Contd.)
 APPROACH – 1
 SELECT location FROM employee WHERE name = ‘A’ ;
 SELECT name FROM employee WHERE location = [x];
 APPROACH – 2
 NESTED QUERY
 SELECT name FROM employee WHERE location IN( SELECT location FROM employee
WHERE name = ‘A’);
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31. JOINS – SELF JOIN(Contd.)
 APPROACH – 3
 SELF JOIN
SELECT e1.name
FROM employee e1, employee e2
WHERE e1.location = e2.location
AND e2.name = ‘A’;
 SELF JOIN must have aliases.
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NAME LOCATION NAME LOCATION
A New york A New york
B Canada A New york
C New york A New york
A New york B Canada
B Canada B Canada
C New york B Canada
A New york C New york
B Canada C New york
C New york C New york
e1 e2

32. TRIGGERS
 A trigger is a statement that the system executes automatically as a side
effect of modification to the database.
 SYNTAX FOR TRIGGER CREATION
DELIMITER //
CREATE TRIGGER `insertmasterproducttax` AFTER INSERT ON `producttax`
FOR EACH ROW BEGIN
INSERT INTO transaction.producttax
SET
transaction.producttax.id = NEW.id,
transaction.producttax.product_id = NEW.product_id,
transaction.producttax.taxtype_id = NEW.taxtype_id;
END;
//
DELIMITER;
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33. MYSQL STORAGE ENGINE
 InnoDB – foreign key constraint
transaction support
ACID properties support
 MYISAM – no foreign key constraint
no transaction support
 Another big difference is concurrency, MYISAM supports table level locking
whereas InnoDB supports row level locking
 With MYISAM, a DML statement will obtain an exclusive lock on the table,
and while that lock is held, no other session can perform a SELECT or a
DML operation on that table.
 InnoDB is slow but supports commit, savepoint and rollback operations
whereas MYISAM is fast but does not support any of these operations.
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34. CURSORS
 cursor is a database objects to retrieve data from a result set one row
at a time, instead of the T-SQL commands that operate on all the
rows in the result set at one time. We use cursor when we need to
update records in a database table in singleton fashion means row by
row.
 Cursor acts as an iterator over a collection of rows in the result set.
 The select statement can return multiple rows as a query result. If you
want to process single row at a time then you can define an explicit
cursor for this select statement.
 Explicit cursors are memory areas which acts as a handle or pointer to
context area and allows to fetch and process query results row by
row.
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35. CURSORS(Contd.)
 Each time you fetch a row from the cursor, it results in a network
roundtrip, whereas as a normal SELECT query makes only one round
trip, however large the result set is.
 Normal select Query fetches all rows in one go while cursor
fetches one row at a time.
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36. CURSORS(Contd.)
 If you have to give a flat hike to your employees using the following
criteria:
 Salary between 30000 and 40000 -- 5000 hike
 Salary between 40000 and 55000 -- 7000 hike
 Salary between 55000 and 65000 -- 9000 hike
 In this situation many developers tend to use a cursor, determine
each employee’s salary and update his salary according to the above
formula.
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37. FLOWCHART OF CURSOR
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38. MULTIPLE UPDATE STATEMENTS
UPDATE [tablename] SET salary =
CASE WHEN salary BETWEEN 30000 AND 40000 THEN salary + 5000
WHEN salary BETWEEN 40000 AND 55000 THEN salary + 7000
WHEN salary BETWEEN 55000 AND 65000 THEN salary + 10000
END
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39. CURSORS - disadvantages
 A cursor is a memory resident set of pointers -- meaning it occupies
memory from your system that may be available for other processes.
Poorly written cursors can completely deplete available memory.
 you must release the resources that are allocated to the cursor
variables.
 A cursor allow us to use set of result-sets returned by mysql query in
one by one pattern. With the use of cursor we can perform
operations on set of resultset on each returned row.Like you are
fetching multiple data by any operation and you want to operate
those data in loop. so with the use of cursor you can loops through
the results.Cursor is convenient to use when you are performing on a
complex result-set.
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