Explaining basic concepts and terminology related to MySQL

1. MYSQL
AGGREGATE FUNCTIONS ,GROUP
BY CLAUSE, JOINS, CONSTRAINTS

2. AGGREGAT
E
FUNCTION
S

3. S.no. Function Purpose
1 MAX() Returns the maximum of
the values under the
specified
column/expression
2 MIN() Returns the minimum of
the values under the
specified
column/expression
3 AVG() Returns the AVERAGE of
the values under the
specified
column/expression
4 SUM() Returns the SUM() of the
values under the specified
column/expression
5 COUNT(
)
Returns the COUNT of the
number of values under
AGGREGAT
E
FUNCTION
S

4. Purpose Statement
To find the highest cost of
any type of shoe in the
factory.
SELECT MAX(cost) FROM
shoes;
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Purpose Statement
To find the lowest cost of
any type of shoe in the
factory.
SELECT MIN(cost) FROM
shoes;
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Purpose Statement
To find the average margin
from shoes table.
SELECT AVG(margin)
FROM shoes;

5. Purpose Statement
To find the total quantity
present in the stock
SELECT SUM(qty) FROM
shoes;
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Purpose Statement
To count the total number
of records in table shoes.
SELECT COUNT(*) FROM
shoes;
To count the different types
of shoes that factory
produces.
SELECT COUNT(distinct
type) FROM shoes;
To count the records for
which the margin is greater
than 2.00
SELECT COUNT(margin)
FROM shoes WHERE
margin>2;
Note- NULL is simply ignored by all the aggregate functions.

6. GROUP BY
CLAUSE
GROUP BY clause is used with the SELECT statement to
divide the rows in a table into group.
Example- To display the total quantity of shoes of various
types.
SELECT type , SUM(qty) FROM shoes GROUP BY type;
Type SUM(qty)
Office 1100
School 7180
Sports 1740
and the output will be

7.  Example- To find the total salary of males and
females working in each department.
SELECT dept,sex,sum(sal)
FROM emp GROUP BY dept,sex;
Department sex salary
RND F 35700.00
RND M 50836.50
Servicing M 27060.00
Marketing F 18150.00

8. H
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 It can be used to restrict groups.
 Rows are grouped.
 Group functions are applied to group.
 Groups that match the criteria in the having clause are
displayed
example- To display the total quantity of shoes of
various types for only those groups for which total
quantity is more than 1500.
SELECT type , SUM(qty) FROM shoes GROUP BY type
HAVING SUM(qty)>1500;
Type SUM(qty)
School 7180
Sports 1748

9. V
S
H
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G
WHERE Conditions are applicable on individual rows whereas HAVING
conditions are applicable on groups formed by GROUP BY clause.
Example- To display total quantity of shoes, of sizes other than 6, of various
types. This report is required only for those groups for which the total
quantity is more than 1500.
SELECT type, SUM(qty) FROM shoes WHERE size <> 6 GROUP BY type
HAVING sum(qty) > 1500;
Type SUM(qty)
School 3780

10. D
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J
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S
A JOIN is a query that combines rows from two or more
tables.
Types of JOINS-
1. Cartesian Product or Cross Join
2. Equi Join
3. Non Equi Join
4. Natural Join

11. C
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S
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P
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S
J
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Cross join of two tables is a table obtained by
pairing up each row of one table with each row
of the other table.
If a table T1 contains 3 rows and 2 columns. A
table t2 contains 4 rows and 3 columns, then
their cartesian product will contain 12 (3*4)
rows and 5 (2+3) columns.

12. I
J
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The Join, in which columns are compared for
equality, is called Equi-Join.
Example- There are three table
Product Table Order Table
Supplier table
To find product details corresponding to each order details,
Select * From Order , Product Where p_code=code;
Code Name Ord_no P_code Sup_code
Supp_code Name Address
Joining
Condition

13. Q
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J
O
I
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A non equi join is a query that specifies some
relationship other than equality between
columns.
Table1- EMP Table2- salgrade
To display emp details along with his grade .
Select EMP.* ,Grade From EMP,salgrade where
salary between losal and hisal;
Empno Name Job salary hiredate Grade Losal Hisal
Non Equi Join Condition

14. NATURAL JOIN
By definition, result of equi join contains two
identical columns. One of the two identical
columns can be eliminated by restating the
query. This result is called a natural join.
Table 1- Emp Table 2- Dept
Select Emp.*, Dname, Loc From Emp, Dept Where
Emp.Dno=Dept.Dno;
Empno Nam
e
Jo
b
salary Dno hiredate Dno Dname Loc

15. FOREIGN KEY
It is a non key attribute which refers to the
primary key of another table.
A foreign key is used to ensure referential
integrity and to get Equi join of two tables.
Product Table Order Table
Supplier table
•Product table- Primary Key (Code)
•Supplier table- Primary key(Supp_code )
•Order table - Ord_no (primary key)
P_code is Foreign key referring to Product(Code)
Sup_code is forengn key referring to Supplier(Supp_code))
Code Name Ord_no P_code Sup_code
Supp_code Name Address

16. REFERENTIAL INTEGRITY
This property of a relational database which
ensures that no entry in a foreign key column
of a table can be made unless it matches a
primary key value in a corresponding related
table is called Referential Integrity.

17. Constraint Guidelines
 Name a constraint or the Oracle Server will
generate a name by using the SYS_Cn format.
 Create a constraint:
 At the same time as the table is created
 After the table has been created
 Define a constraint at the column or table level.
 View a constraint in the data dictionary.

18. The NOT NULL Constraint
 Defined at the column level
SQL> CREATE TABLE emp(
2 empno NUMBER(4),
3 ename VARCHAR2(10) NOT NULL,
4 job VARCHAR2(9),
5 mgr NUMBER(4),
6 hiredate DATE,
7 sal NUMBER(7,2),
8 comm NUMBER(7,2),
9 deptno NUMBER(7,2) NOT NULL);

19. The PRIMARY KEY Constraint
 Defined at either the table level or the column
level
SQL> CREATE TABLE dept(
2 deptno NUMBER(2),
3 dname VARCHAR2(14),
4 loc VARCHAR2(13),
5 CONSTRAINT dept_dname_uk UNIQUE (dname),
6 CONSTRAINT dept_deptno_pk PRIMARY KEY(deptno));

20. The FOREIGN KEY Constraint
 Defined at either the table level or the
column level
SQL> CREATE TABLE emp(
2 empno NUMBER(4),
3 ename VARCHAR2(10) NOT NULL,
4 job VARCHAR2(9),
5 mgr NUMBER(4),
6 hiredate DATE,
7 sal NUMBER(7,2),
8 comm NUMBER(7,2),
9 deptno NUMBER(7,2) NOT NULL,
10 CONSTRAINT emp_deptno_fk FOREIGN KEY (deptno)
11 REFERENCES dept (deptno));

21. The CHECK Constraint
 Defines a condition that each row must satisfy
 Expressions that are not allowed:
 References to CURRVAL, NEXTVAL, LEVEL, and
ROWNUM pseudocolumns
 Calls to SYSDATE, UID, USER, and USERENV
functions
 Queries that refer to other values in other rows
..., deptno NUMBER(2),
CONSTRAINT emp_deptno_ck
CHECK (DEPTNO BETWEEN 10 AND 99),...

22. Adding a Constraint
 Add a FOREIGN KEY constraint to the EMP
table indicating that a manager must already
exist as a valid employee in the EMP table.
SQL> ALTER TABLE emp
2 ADD CONSTRAINT emp_mgr_fk
3 FOREIGN KEY(mgr) REFERENCES emp(empno);
Table altered.

23. Dropping a Constraint
 Remove the manager constraint from the EMP
table.
SQL> ALTER TABLE emp
2 DROP CONSTRAINT emp_mgr_fk;
Table altered.
• Remove the PRIMARY KEY constraint on the DEPT table and drop
the associated FOREIGN KEY constraint on the EMP.DEPTNO
column.
SQL> ALTER TABLE dept
2 DROP PRIMARY KEY CASCADE;
Table altered.

24. Disabling Constraints
 Execute the DISABLE clause of the ALTER
TABLE statement to deactivate an integrity
constraint.
 Apply the CASCADE option to disable
dependent integrity constraints.
SQL> ALTER TABLE emp
2 DISABLE CONSTRAINT emp_empno_pk CASCADE;
Table altered.

25. Enabling Constraints
 Activate an integrity constraint currently
disabled in the table definition by using the
ENABLE clause.
 A UNIQUE or PRIMARY KEY index is
automatically created if you enable a UNIQUE
key or PRIMARY KEY constraint.
SQL> ALTER TABLE emp
2 ENABLE CONSTRAINT emp_empno_pk;
Table altered.