Microsoft.NET

……………………………………………….Expertise in .NET Technologies

Important functions in Oracle – Part XIII

Posted by Ravi Varma Thumati on October 15, 2009

COUNT Function

The COUNT function returns the number of rows in a query.

The syntax for the COUNT function is:

SELECT COUNT(expression)
FROM tables
WHERE predicates;

Note:

The COUNT function will only count those records in which the field in the brackets is NOT NULL.

For example, if you have the following table called suppliers:

Supplier_ID

Supplier_Name

State

1 IBM CA
2 Microsoft
3 NVIDIA

The result for this query will return 3.

Select COUNT(Supplier_ID) from suppliers;

While the result for the next query will only return 1, since there is only one row in the suppliers table where the State field is NOT NULL.

Select COUNT(State) from suppliers;

Simple Example

For example, you might wish to know how many employees have a salary that is above $25,000 / year.

SELECT COUNT(*) as “Number of employees” FROM employees WHERE salary > 25000;

In this example, we’ve aliased the count(*) field as “Number of employees”. As a result, “Number of employees” will display as the field name when the result set is returned.

Example using DISTINCT

You can use the DISTINCT clause within the COUNT function.

For example, the SQL statement below returns the number of unique departments where at least one employee makes over $25,000 / year.

SELECT COUNT(DISTINCT department) as “Unique departments” FROM employees
WHERE salary > 25000;

Again, the count(DISTINCT department) field is aliased as “Unique departments”. This is the field name that will display in the result set.

Example using GROUP BY

In some cases, you will be required to use a GROUP BY clause with the COUNT function.

For example, you could use the COUNT function to return the name of the department and the number of employees (in the associated department) that make over $25,000 / year.

SELECT department, COUNT(*) as “Number of employees”
FROM employees WHERE salary > 25000
GROUP BY department;

Because you have listed one column in your SELECT statement that is not encapsulated in the COUNT function, you must use a GROUP BY clause. The department field must, therefore, be listed in the GROUP BY section.

TIP: Performance Tuning

Since the COUNT function will return the same results regardless of what NOT NULL field(s) you include as the COUNT function parameters (ie: within the brackets), you can change the syntax of the COUNT function to COUNT(1) to get better performance as the database engine will not have to fetch back the data fields.

For example, based on the example above, the following syntax would result in better performance:

SELECT department, COUNT(1) as “Number of employees”
FROM employees WHERE salary > 25000
GROUP BY department;

Now, the COUNT function does not need to retrieve all fields from the employees table as it had to when you used the COUNT(*) syntax. It will merely retrieve the numeric value of 1 for each record that meets your criteria.

Practice Exercise #1:

Based on the employees table populated with the following data, count the number of employees whose salary is over $55,000 per year.

CREATE TABLE employees

(  employee_number   number(10)          not null,

employee_name       varchar2(50)        not null,

salary                        number(6),

CONSTRAINT employees_pk PRIMARY KEY (employee_number)

);

INSERT INTO employees (employee_number, employee_name, salary)
VALUES (1001, ‘John Smith’, 62000);

INSERT INTO employees (employee_number, employee_name, salary)
VALUES (1002, ‘Jane Anderson’, 57500);

INSERT INTO employees (employee_number, employee_name, salary)
VALUES (1003, ‘Brad Everest’, 71000);

INSERT INTO employees (employee_number, employee_name, salary)
VALUES (1004, ‘Jack Horvath’, 42000);

Solution:

Although inefficient in terms of performance, the following SQL statement would return the number of employees whose salary is over $55,000 per year.

SELECT COUNT(*) as “Number of employees” FROM employees WHERE salary > 55000;

It would return the following result set:

Number of employees

3

A more efficient implementation of the same solution would be the following SQL statement:

SELECT COUNT(1) as “Number of employees” FROM employees
WHERE salary > 55000;

Now, the COUNT function does not need to retrieve all of the fields from the table (ie: employee_number, employee_name, and salary), but rather whenever the condition is met, it will retrieve the numeric value of 1. Thus, increasing the performance of the SQL statement.

Practice Exercise #2:

Based on the suppliers table populated with the following data, count the number of distinct cities in the suppliers table:

CREATE TABLE suppliers

(  supplier_id              number(10)          not null,

supplier_name       varchar2(50)        not null,

city                          varchar2(50),

CONSTRAINT suppliers_pk PRIMARY KEY (supplier_id)

);

INSERT INTO suppliers (supplier_id, supplier_name, city)
VALUES (5001, ‘Microsoft’, ‘New York’);

INSERT INTO suppliers (supplier_id, supplier_name, city)
VALUES (5002, ‘IBM’, ‘Chicago’);

INSERT INTO suppliers (supplier_id, supplier_name, city)
VALUES (5003, ‘Red Hat’, ‘Detroit’);

INSERT INTO suppliers (supplier_id, supplier_name, city)
VALUES (5004, ‘NVIDIA’, ‘New York’);

INSERT INTO suppliers (supplier_id, supplier_name, city)
VALUES (5005, ‘NVIDIA’, ‘LA’);

Solution:

The following SQL statement would return the number of distinct cities in the suppliers table:

SELECT COUNT(DISTINCT city) as “Distinct Cities” FROM suppliers;

It would return the following result set:

Distinct Cities

4

Practice Exercise #3:

Based on the customers table populated with the following data, count the number of distinct cities for each customer_name in the customers table:

CREATE TABLE customers

(  customer_id            number(10)          not null,

customer_name     varchar2(50)        not null,

city                           varchar2(50),

CONSTRAINT customers_pk PRIMARY KEY (customer_id)

);

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7001, ‘Microsoft’, ‘New York’);

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7002, ‘IBM’, ‘Chicago’);

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7003, ‘Red Hat’, ‘Detroit’);

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7004, ‘Red Hat’, ‘New York’);

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7005, ‘Red Hat’, ‘San Francisco’);

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7006, ‘NVIDIA’, ‘New York’);

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7007, ‘NVIDIA’, ‘LA’);

INSERT INTO customers (customer_id, customer_name, city)
VALUES (7008, ‘NVIDIA’, ‘LA’);

Solution:

The following SQL statement would return the number of distinct cities for each customer_name in the customers table:

SELECT customer_name, COUNT(DISTINCT city) as “Distinct Cities”
FROM customers
GROUP BY customer_name;

It would return the following result set:

CUSTOMER_NAME

Distinct Cities

IBM 1
Microsoft 1
NVIDIA 2
Red Hat 3

SUM Function

The SUM function returns the summed value of an expression.

The syntax for the SUM function is:

SELECT SUM(expression )
FROM tables
WHERE predicates;

expression can be a numeric field or formula.

Simple Example

For example, you might wish to know how the combined total salary of all employees whose salary is above $25,000 / year.

SELECT SUM(salary) as “Total Salary” FROM employees WHERE salary > 25000;

In this example, we’ve aliased the sum(salary) field as “Total Salary”. As a result, “Total Salary” will display as the field name when the result set is returned.

Example using DISTINCT

You can use the DISTINCT clause within the SUM function. For example, the SQL statement below returns the combined total salary of unique salary values where the salary is above $25,000 / year.

SELECT SUM(DISTINCT salary) as “Total Salary” FROM employees

WHERE salary > 25000;

If there were two salaries of $30,000/year, only one of these values would be used in the SUM function.

Example using a Formula

The expression contained within the SUM function does not need to be a single field. You could also use a formula. For example, you might want the net income for a business. Net Income is calculated as total income less total expenses.

SELECT SUM(income – expenses) as “Net Income” FROM gl_transactions;

You might also want to perform a mathematical operation within a SUM function. For example, you might determine total commission as 10% of total sales.

SELECT SUM(sales * 0.10) as “Commission”
FROM order_details;

Example using GROUP BY

In some cases, you will be required to use a GROUP BY clause with the SUM function.

For example, you could also use the SUM function to return the name of the department and the total sales (in the associated department).

SELECT department, SUM(sales) as “Total sales” FROM order_details GROUP BY department;

Because you have listed one column in your SELECT statement that is not encapsulated in the SUM function, you must use a GROUP BY clause. The department field must, therefore, be listed in the GROUP BY section.

MIN Function

The MIN function returns the minimum value of an expression.

The syntax for the MIN function is:

SELECT MIN(expression )
FROM tables
WHERE predicates;

Simple Example

For example, you might wish to know the minimum salary of all employees.

SELECT MIN(salary) as “Lowest salary” FROM employees;

In this example, we’ve aliased the min(salary) field as “Lowest salary”. As a result, “Lowest salary” will display as the field name when the result set is returned.

Example using GROUP BY

In some cases, you will be required to use a GROUP BY clause with the MIN function.

For example, you could also use the MIN function to return the name of each department and the minimum salary in the department.

SELECT department, MIN(salary) as “Lowest salary”
FROM employees
GROUP BY department;

Because you have listed one column in your SELECT statement that is not encapsulated in the MIN function, you must use a GROUP BY clause. The department field must, therefore, be listed in the GROUP BY section.

MAX Function

The MAX function returns the maximum value of an expression.

The syntax for the MAX function is:

SELECT MAX(expression )
FROM tables
WHERE predicates;

Simple Example

For example, you might wish to know the maximum salary of all employees.

SELECT MAX(salary) as “Highest salary” FROM employees;

In this example, we’ve aliased the max(salary) field as “Highest salary”. As a result, “Highest salary” will display as the field name when the result set is returned.

Example using GROUP BY

In some cases, you will be required to use a GROUP BY clause with the MAX function.

For example, you could also use the MAX function to return the name of each department and the maximum salary in the department.

SELECT department, MAX(salary) as “Highest salary” FROM employees
GROUP BY department;

Because you have listed one column in your SELECT statement that is not encapsulated in the MAX function, you must use a GROUP BY clause. The department field must, therefore, be listed in the GROUP BY section.

WHERE Clause

The WHERE clause allows you to filter the results from an SQL statement – select, insert, update, or delete statement.

It is difficult to explain the basic syntax for the WHERE clause, so instead, we’ll take a look at some examples.

Example #1

SELECT * FROM suppliers WHERE supplier_name = ‘IBM’;

In this first example, we’ve used the WHERE clause to filter our results from the suppliers table. The SQL statement above would return all rows from the suppliers table where the supplier_name is IBM. Because the * is used in the select, all fields from the suppliers table would appear in the result set.

Example #2

SELECT supplier_id
FROM suppliers
WHERE supplier_name = ‘IBM’
or supplier_city = ‘Newark’;

We can define a WHERE clause with multiple conditions. This SQL statement would return all supplier_id values where the supplier_name is IBM or the supplier_city is Newark.

Example #3

SELECT suppliers.suppler_name, orders.order_id
FROM suppliers, orders
WHERE suppliers.supplier_id = orders.supplier_id
and suppliers.supplier_city = ‘Atlantic City’;

We can also use the WHERE clause to join multiple tables together in a single SQL statement. This SQL statement would return all supplier names and order_ids where there is a matching record in the suppliers and orders tables based on supplier_id, and where the supplier_city is Atlantic City.

“AND” Condition

The AND condition allows you to create an SQL statement based on 2 or more conditions being met. It can be used in any valid SQL statement – select, insert, update, or delete.

The syntax for the AND condition is:

SELECT columns FROM tables WHERE column1 = ‘value1’ and column2 = ‘value2’;

The AND condition requires that each condition be must be met for the record to be included in the result set. In this case, column1 has to equal ‘value1’ and column2 has to equal ‘value2’.

Example #1

The first example that we’ll take a look at involves a very simple example using the AND condition.

SELECT * FROM suppliers WHERE city = ‘New York’ and type = ‘PC Manufacturer’;

This would return all suppliers that reside in New York and are PC Manufacturers. Because the * is used in the select, all fields from the supplier table would appear in the result set.

Example #2

Our next example demonstrates how the AND condition can be used to “join” multiple tables in an SQL statement.

SELECT orders.order_id, suppliers.supplier_name
FROM suppliers, orders
WHERE suppliers.supplier_id = orders.supplier_id
and suppliers.supplier_name = ‘IBM’;

This would return all rows where the supplier_name is IBM. And the suppliers and orders tables are joined on supplier_id. You will notice that all of the fields are prefixed with the table names (ie: orders.order_id). This is required to eliminate any ambiguity as to which field is being referenced; as the same field name can exist in both the suppliers and orders tables.

In this case, the result set would only display the order_id and supplier_name fields (as listed in the first part of the select statement.).

“OR” Condition

The OR condition allows you to create an SQL statement where records are returned when any one of the conditions are met. It can be used in any valid SQL statement – select, insert, update, or delete.

The syntax for the OR condition is:

SELECT columns FROM tables WHERE column1 = ‘value1’ or column2 = ‘value2’;

The OR condition requires that any of the conditions be must be met for the record to be included in the result set. In this case, column1 has to equal ‘value1’ OR column2 has to equal ‘value2’.

Example #1

The first example that we’ll take a look at involves a very simple example using the OR condition.

SELECT * FROM suppliers WHERE city = ‘New York’ or city = ‘Newark’;

This would return all suppliers that reside in either New York or Newark. Because the * is used in the select, all fields from the suppliers table would appear in the result set.

Example #2

The next example takes a look at three conditions. If any of these conditions is met, the record will be included in the result set.

SELECT supplier_id
FROM suppliers
WHERE name = ‘IBM’
or name = ‘Hewlett Packard’
or name = ‘Gateway’;

This SQL statement would return all supplier_id values where the supplier’s name is either IBM, Hewlett Packard or Gateway.

Combining the “AND” and “OR” Conditions

The AND and OR conditions can be combined in a single SQL statement. It can be used in any valid SQL statement – select, insert, update, or delete.

When combining these conditions, it is important to use brackets so that the database knows what order to evaluate each condition.

Example #1

The first example that we’ll take a look at an example that combines the AND and OR conditions.

SELECT *
FROM suppliers
WHERE (city = ‘New York’ and name = ‘IBM’)
or (city = ‘Newark’);

This would return all suppliers that reside in New York whose name is IBM and all suppliers that reside in Newark. The brackets determine what order the AND and OR conditions are evaluated in.

Example #2

The next example takes a look at a more complex statement.

For example:

SELECT supplier_id
FROM suppliers
WHERE (name = ‘IBM’)
or (name = ‘Hewlett Packard’ and city = ‘Atlantic City’)
or (name = ‘Gateway’ and status = ‘Active’ and city = ‘Burma’);

This SQL statement would return all supplier_id values where the supplier’s name is IBM or the name is Hewlett Packard and the city is Atlantic City or the name is Gateway, the status is Active, and the city is Burma.

LIKE Condition

The LIKE condition allows you to use wildcards in the where clause of an SQL statement. This allows you to perform pattern matching. The LIKE condition can be used in any valid SQL statement – select, insert, update, or delete.

The patterns that you can choose from are:

% allows you to match any string of any length (including zero length)

_ allows you to match on a single character

Examples using % wildcard

The first example that we’ll take a look at involves using % in the where clause of a select statement. We are going to try to find all of the suppliers whose name begins with ‘Hew’.

SELECT * FROM suppliers WHERE supplier_name like ‘Hew%’;

You can also using the wildcard multiple times within the same string. For example,

SELECT * FROM suppliers WHERE supplier_name like ‘%bob%’;

In this example, we are looking for all suppliers whose name contains the characters ‘bob’.

You could also use the LIKE condition to find suppliers whose name does not start with ‘T’. For example,

SELECT * FROM suppliers
WHERE supplier_name not like ‘T%’;

By placing the not keyword in front of the LIKE condition, you are able to retrieve all suppliers whose name does not start with ‘T’.

Examples using _ wildcard

Next, let’s explain how the _ wildcard works. Remember that the _ is looking for only one character.

For example,

SELECT * FROM suppliers WHERE supplier_name like ‘Sm_th’;

This SQL statement would return all suppliers whose name is 5 characters long, where the first two characters is ‘Sm’ and the last two characters is ‘th’. For example, it could return suppliers whose name is ‘Smith’, ‘Smyth’, ‘Smath’, ‘Smeth’, etc.

Here is another example,

SELECT * FROM suppliers WHERE account_number like ‘12317_’;

You might find that you are looking for an account number, but you only have 5 of the 6 digits. The example above, would retrieve potentially 10 records back (where the missing value could equal anything from 0 to 9). For example, it could return suppliers whose account numbers are:

123170
123171
123172
123173
123174
123175
123176
123177
123178
123179.

Examples using Escape Characters

Next, in Oracle, let’s say you wanted to search for a % or a _ character in a LIKE condition. You can do this using an Escape character.

Please note that you can define an escape character as a single character (length of 1) ONLY.

For example,

SELECT * FROM suppliers WHERE supplier_name LIKE ‘!%’ escape ‘!’;

This SQL statement identifies the ! character as an escape character. This statement will return all suppliers whose name is %.

Here is another more complicated example:

SELECT * FROM suppliers WHERE supplier_name LIKE ‘H%!%’ escape ‘!’;

This example returns all suppliers whose name starts with H and ends in %. For example, it would return a value such as ‘Hello%’.

You can also use the Escape character with the _ character. For example,

SELECT * FROM suppliers WHERE supplier_name LIKE ‘H%!_’ escape ‘!’;

This example returns all suppliers whose name starts with H and ends in _. For example, it would return a value such as ‘Hello_’.

“IN” Function

The IN function helps reduce the need to use multiple OR conditions.

The syntax for the IN function is:

SELECT columns
FROM tables
WHERE column1 in (value1, value2, …. value_n);

This SQL statement will return the records where column1 is value1, value2…, or value_n. The IN function can be used in any valid SQL statement – select, insert, update, or delete.

Example #1

The following is an SQL statement that uses the IN function:

SELECT *
FROM suppliers
WHERE supplier_name in ( ‘IBM’, ‘Hewlett Packard’, ‘Microsoft’);

This would return all rows where the supplier_name is either IBM, Hewlett Packard, or Microsoft. Because the * is used in the select, all fields from the suppliers table would appear in the result set.

It is equivalent to the following statement:

SELECT *
FROM suppliers
WHERE supplier_name = ‘IBM’
OR supplier_name = ‘Hewlett Packard’
OR supplier_name = ‘Microsoft’;

As you can see, using the IN function makes the statement easier to read and more efficient.

Example #2

You can also use the IN function with numeric values.

SELECT *
FROM orders
WHERE order_id in (10000, 10001, 10003, 10005);

This SQL statement would return all orders where the order_id is either 10000, 10001, 10003, or 10005.

It is equivalent to the following statement:

SELECT * FROM orders WHERE order_id = 10000 OR order_id = 10001
OR order_id = 10003 OR order_id = 10005;

Example #3 using “NOT IN”

The IN function can also be combined with the NOT operator.

For example,

SELECT *
FROM suppliers
WHERE supplier_name not in ( ‘IBM’, ‘Hewlett Packard’, ‘Microsoft’);

This would return all rows where the supplier_name is neither IBM, Hewlett Packard, or Microsoft. Sometimes, it is more efficient to list the values that you do not want, as opposed to the values that you do want.

BETWEEN Condition

The BETWEEN condition allows you to retrieve values within a range.

The syntax for the BETWEEN condition is:

SELECT columns FROM tables
WHERE column1 between value1 and value2;

This SQL statement will return the records where column1 is within the range of value1 and value2 (inclusive). The BETWEEN function can be used in any valid SQL statement – select, insert, update, or delete.

Example #1 – Numbers

The following is an SQL statement that uses the BETWEEN function:

SELECT * FROM suppliers
WHERE supplier_id between 5000 AND 5010;

This would return all rows where the supplier_id is between 5000 and 5010, inclusive. It is equivalent to the following SQL statement:

SELECT * FROM suppliers WHERE supplier_id >= 5000 AND supplier_id <= 5010;

Example #2 – Dates

You can also use the BETWEEN function with dates.

SELECT * FROM orders
WHERE order_date between to_date (‘2003/01/01’, ‘yyyy/mm/dd’)
AND to_date (‘2003/12/31’, ‘yyyy/mm/dd’);

This SQL statement would return all orders where the order_date is between Jan 1, 2003 and Dec 31, 2003 (inclusive).

It would be equivalent to the following SQL statement:

SELECT *
FROM orders
WHERE order_date >= to_date(‘2003/01/01’, ‘yyyy/mm/dd’)
AND order_date <= to_date(‘2003/12/31′,’yyyy/mm/dd’);

Example #3 – NOT BETWEEN

The BETWEEN function can also be combined with the NOT operator.

For example,

SELECT * FROM suppliers WHERE supplier_id not between 5000 and 5500;

This would be equivalent to the following SQL:

SELECT * FROM suppliers WHERE supplier_id < 5000 OR supplier_id > 5500;

In this example, the result set would exclude all supplier_id values between the range of 5000 and 5500 (inclusive).

EXISTS Condition

The EXISTS condition is considered “to be met” if the subquery returns at least one row.

The syntax for the EXISTS condition is:

SELECT columns
FROM tables
WHERE EXISTS ( subquery );

The EXISTS condition can be used in any valid SQL statement – select, insert, update, or delete.

Example #1

Let’s take a look at a simple example. The following is an SQL statement that uses the EXISTS condition:

SELECT *
FROM suppliers
WHERE EXISTS
(select * from orders
where suppliers.supplier_id = orders.supplier_id);

This select statement will return all records from the suppliers table where there is at least one record in the orders table with the same supplier_id.

Example #2 – NOT EXISTS

The EXISTS condition can also be combined with the NOT operator.

For example,

SELECT *
FROM suppliers
WHERE not exists (select * from orders Where suppliers.supplier_id = orders.supplier_id);

This will return all records from the suppliers table where there are no records in the orders table for the given supplier_id.

Example #3 – DELETE Statement

The following is an example of a delete statement that utilizes the EXISTS condition:

DELETE FROM suppliers
WHERE EXISTS
(select *
from orders
where suppliers.supplier_id = orders.supplier_id);

Example #4 – UPDATE Statement

The following is an example of an update statement that utilizes the EXISTS condition:

UPDATE suppliers

SET supplier_name =            ( SELECT customers.name
FROM customers
WHERE customers.customer_id = suppliers.supplier_id)

WHERE EXISTS
( SELECT customers.name
FROM customers
WHERE customers.customer_id = suppliers.supplier_id);

Example #5 – INSERT Statement

The following is an example of an insert statement that utilizes the EXISTS condition:

INSERT INTO suppliers
(supplier_id, supplier_name)
SELECT account_no, name
FROM suppliers
WHERE exists (select * from orders Where suppliers.supplier_id = orders.supplier_id);

GROUP BY Clause

The GROUP BY clause can be used in a SELECT statement to collect data across multiple records and group the results by one or more columns.

The syntax for the GROUP BY clause is:

SELECT column1, column2, … column_n, aggregate_function (expression)
FROM tables
WHERE predicates
GROUP BY column1, column2, … column_n;

Example using the SUM function

For example, you could also use the SUM function to return the name of the department and the total sales (in the associated department).

SELECT department, SUM(sales) as “Total sales”
FROM order_details
GROUP BY department;

Because you have listed one column in your SELECT statement that is not encapsulated in the SUM function, you must use a GROUP BY clause. The department field must, therefore, be listed in the GROUP BY section.

Example using the COUNT function

For example, you could use the COUNT function to return the name of the department and the number of employees (in the associated department) that make over $25,000 / year.

SELECT department, COUNT(*) as “Number of employees”
FROM employees
WHERE salary > 25000
GROUP BY department;

Example using the MIN function

For example, you could also use the MIN function to return the name of each department and the minimum salary in the department.

SELECT department, MIN(salary) as “Lowest salary”
FROM employees
GROUP BY department;

Example using the MAX function

For example, you could also use the MAX function to return the name of each department and the maximum salary in the department.

SELECT department, MAX(salary) as “Highest salary”
FROM employees
GROUP BY department;

HAVING Clause

The HAVING clause is used in combination with the GROUP BY clause. It can be used in a SELECT statement to filter the records that a GROUP BY returns.

The syntax for the HAVING clause is:

SELECT column1, column2, … column_n, aggregate_function (expression)
FROM tables
WHERE predicates
GROUP BY column1, column2, … column_n
HAVING condition1 … condition_n;

Example using the SUM function

For example, you could also use the SUM function to return the name of the department and the total sales (in the associated department). The HAVING clause will filter the results so that only departments with sales greater than $1000 will be returned.

SELECT department, SUM(sales) as “Total sales”
FROM order_details
GROUP BY department
HAVING SUM(sales) > 1000;

Example using the COUNT function

For example, you could use the COUNT function to return the name of the department and the number of employees (in the associated department) that make over $25,000 / year. The HAVING clause will filter the results so that only departments with more than 10 employees will be returned.

SELECT department, COUNT(*) as “Number of employees”
FROM employees
WHERE salary > 25000
GROUP BY department
HAVING COUNT(*) > 10;

Example using the MIN function

For example, you could also use the MIN function to return the name of each department and the minimum salary in the department. The HAVING clause will return only those departments where the starting salary is $35,000.

SELECT department, MIN(salary) as “Lowest salary”
FROM employees
GROUP BY department
HAVING MIN(salary) = 35000;

Example using the MAX function

For example, you could also use the MAX function to return the name of each department and the maximum salary in the department. The HAVING clause will return only those departments whose maximum salary is less than $50,000.

SELECT department, MAX(salary) as “Highest salary”
FROM employees
GROUP BY department
HAVING MAX(salary) < 50000;

ORDER BY Clause

The ORDER BY clause allows you to sort the records in your result set. The ORDER BY clause can only be used in SELECT statements.

The syntax for the ORDER BY clause is:

SELECT columns FROM tables
WHERE predicates ORDER BY column ASC/DESC;

The ORDER BY clause sorts the result set based on the columns specified. If the ASC or DESC value is omitted, it is sorted by ASC.

ASC indicates ascending order. (default)
DESC indicates descending order.

Example #1

SELECT supplier_city FROM suppliers WHERE supplier_name = ‘IBM’ ORDER BY supplier_city;

This would return all records sorted by the supplier_city field in ascending order.

Example #2

SELECT supplier_city FROM suppliers
WHERE supplier_name = ‘IBM’ ORDER BY supplier_city DESC;

This would return all records sorted by the supplier_city field in descending order.

Example #3

You can also sort by relative position in the result set, where the first field in the result set is 1. The next field is 2, and so on.

SELECT supplier_city FROM suppliers WHERE supplier_name = ‘IBM’

ORDER BY 1 DESC;

This would return all records sorted by the supplier_city field in descending order, since the supplier_city field is in position #1 in the result set.

Example #4

SELECT supplier_city, supplier_state FROM suppliers
WHERE supplier_name = ‘IBM’ ORDER BY supplier_city DESC, supplier_state ASC;

This would return all records sorted by the supplier_city field in descending order, with a secondary sort by supplier_state in ascending order.

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