Query with SELECT: Introduction

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Syntax for SELECT ... FROM Expressions in the select list Alias

A query is a request for retrieval of data from the database. In SQL, queries are performed using the SELECT command.

In the following examples, the table s6a is used.

Syntax for SELECT ... FROM Top

The syntax for the simplest SELECT statement is as follows:

SELECT <select_list> FROM <table_name>

This statement retrieves the selected fields in <select_list> of all the records in the table named <table_name>. The order of the fields in each record of the output is the same as the order of the fields in <select_list>.

If all the fields are to be retrieved, use "*" for <select_list>.

The order of the records appeared in the output is the same as the order of the records in the table.

For examples:

Retrieve the whole table s6a:
SELECT * FROM s6a
Output:
class_num stud_id name email clc ue phy pm cs
1 92114 Chan Wai Man, Raymond s92114@sample.edu.hk 64 55 70 64 59
2 92133

Chow Chi Ling

 clchow@example.com 70 62 62 59 70
3 94302 Fung Ching Man, Mandy mandyfung@example.com 72 50 42 59 60
4 91131 Hung Wai Ming s91131@sample.edu.hk 52 48 55 39 59
5 92153 Leung King kleung@example.com 40 50 51 40 60
6 92211 Poon Kwok Fai kwokfai@testing.com.hk 59 60 70 77 75
7 91194 Sung Hing Wah, Patrick patricksung@example.com 70 72 81 69 70
8 97602 Tang Wing Chi s97602@sample.edu.hk 80 79 70 72 69
9 92145 Wong Ka Tak, Kent klkwon@example.com 35 40 52 50 42
10 93211 Yeung Chun cyeung@testing.com.hk 69 80 77 60 52
11 96374 Lai Fung Chun s96374@sample.edu.hk 78 75 69 65 61
12 94412 Chan Lai Yin cly@testing.com.hk 43 59 53 61 60
13 98832 Man Fook Wing fwman@testing.com.hk 55 77 34 40 51
14 95343 Chung Kwok Fai, Fred fredchung@example.com 72 62 53 47 50
15 97233 Lee Lai May maylee@testing.com.hk 67 71 56 60 55
 
Select the fields name, stud_id and cs from the table s6a (note the order of the fields):
SELECT name, stud_id, cs FROM s6a
Output:
name stud_id cs
Chan Wai Man, Raymond 92114 59

Chow Chi Ling

 92133 70
Fung Ching Man, Mandy 94302 60
Hung Wai Ming 91131 59
Leung King 92153 60
Poon Kwok Fai 92211 75
Sung Hing Wah, Patrick 91194 70
Tang Wing Chi 97602 69
Wong Ka Tak, Kent 92145 42
Yeung Chun 93211 52
Lai Fung Chun 96374 61
Chan Lai Yin 94412 60
Man Fook Wing 98832 51
Chung Kwok Fai, Fred 95343 50
Lee Lai May 97233 55

Expressions in the select list Top

Apart from a list of fields and "*", the <select_list> after the keyword SELECT can also be a list of expressions. An expression can be a constant, a field names or any combination of the above connected by arithmetic operators (e.g. +, -, *, /) and parentheses.

For example:

List the class numbers, names and average marks of the five subjects of all the students in the class S6A:
SELECT class_num, name, (clc + ue + pm + phy + cs) / 5 FROM s6a
Sample output (system dependent):
class_num name Exp_3
1 Chan Wai Man, Raymond 62.4000
2

Chow Chi Ling

 64.6000
3 Fung Ching Man, Mandy 56.6000
4 Hung Wai Ming 50.6000
5 Leung King 48.2000
6 Poon Kwok Fai 68.2000
7 Sung Hing Wah, Patrick 72.4000
8 Tang Wing Chi 74.0000
9 Wong Ka Tak, Kent 43.8000
10 Yeung Chun 67.6000
11 Lai Fung Chun 69.6000
12 Chan Lai Yin 55.2000
13 Man Fook Wing 51.4000
14 Chung Kwok Fai, Fred 56.8000
15 Lee Lai May 61.8000

In the output, the system will create a "field name" by itself (e.g. Exp_3 as shown above) for an expression appeared in the <select_list>. The display format of the numerical value (in this case, Exp_3) is software dependent.

Alias Top

Sometimes we want to have a more meaningful descriptive name for a field name in the output. This can be achieved by adding the keyword AS and the new name (called alias) after the field name.

For example:

Select the fields name, stud_id and cs from the table s6a, and displaying the field name cs as Computer_studies.
SELECT name, stud_id, cs AS computer_studies FROM s6a
Output:
name stud_id Computer_studies
Chan Wai Man, Raymond 92114 59

Chow Chi Ling

 92133 70
Fung Ching Man, Mandy 94302 60
Hung Wai Ming 91131 59
Leung King 92153 60
Poon Kwok Fai 92211 75
Sung Hing Wah, Patrick 91194 70
Tang Wing Chi 97602 69
Wong Ka Tak, Kent 92145 42
Yeung Chun 93211 52
Lai Fung Chun 96374 61
Chan Lai Yin 94412 60
Man Fook Wing 98832 51
Chung Kwok Fai, Fred 95343 50
Lee Lai May 97233 55

Aliases are often used together with expressions:

List the class numbers, names and average marks of the five subjects of all the students in the class S6A, and displaying the average marks as Average:
SELECT class_num, name, (clc + ue + pm + phy + cs) / 5 AS Average FROM s6a
Sample output (system dependent):
class_num name Average
1 Chan Wai Man, Raymond 62.4000
2

Chow Chi Ling

 64.6000
3 Fung Ching Man, Mandy 56.6000
4 Hung Wai Ming 50.6000
5 Leung King 48.2000
6 Poon Kwok Fai 68.2000
7 Sung Hing Wah, Patrick 72.4000
8 Tang Wing Chi 74.0000
9 Wong Ka Tak, Kent 43.8000
10 Yeung Chun 67.6000
11 Lai Fung Chun 69.6000
12 Chan Lai Yin 55.2000
13 Man Fook Wing 51.4000
14 Chung Kwok Fai, Fred 56.8000
15 Lee Lai May 61.8000

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