One of the main functions of SQL is to get data samples from the DBMS. To do this, SQL uses the SELECT operator. Let's look at a few simple queries with his participation.
To begin with, it is important to understand that using the SELECT operator, you can output data not only from database tables, but also arbitrary rows, numbers, dates, etc. For example, this way you can output an arbitrary string:
SELECT "Hello world"
The * symbol is used to output all fields from a specific table. Let's take a look at the database schema and output the data from one of the tables.
SELECT * FROM FamilyMembers
If it is necessary to display information only on certain columns of the table, and not all at once, then this can be done by listing the column names separated by commas:
SELECT member_id, member_name FROM FamilyMembers
In case we want to output some columns of the table, but so that they are named differently in the final selection, we can use aliases.
Their syntax is quite simple, we have to use the AS operator. As in the example below:
SELECT member_id, member_name AS Name FROM FamilyMembers
Or you can do without it by simply writing the desired field name separated by a space.
SELECT member_id, member_name Name FROM FamilyMembers
Aliases can contain up to 255 characters (including spaces, numbers, and special characters)
This is our first lesson of the practical module. Before that, there were only theoretical ones aimed at filling potential gaps in the theory of relational databases. After each practical lesson, we offer a group of tasks for self-testing work in order to immediately practise the information received.
If you missed the module "Introduction", namely the article "Course structure", which described the principle of operation and interface of the block "Tasks for self-testing", then we recommend to return to it.