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What is Structured Query Language (SQL)?

SQL (Structured Query Language) is a programming language used for managing the data that is stored in a DBMS (DataBase Management System). The Query part in SQL refers to the act of "asking a database" for a certain type of information; you are querying the database.

There are several implementations (software) of DBMS. Each software provides its own query language. For this course, you will learn PostgreSQL.

To learn more, check out the following:

{% hyf-youtube src="https://www.youtube.com/watch?v=kqUIoOM3WEs" %}

What are data types (as applied to databases)?

When you store data in PostgreSQL, each datum (singular of the word data) needs to be associated with its type.

For example numbers like 42, 1636 or -345 are all associated with the type int.

The following is a list of the most frequently used PostgreSQL data types.

Type Description Example Value
INTEGER Numbers 42
REAL Decimal numbers 3.14
VARCHAR(N) String with variable maximum of N characters 'Dragon'
TEXT String with unlimited length 'Positive'
TIMESTAMP Store date and time without timezone 2019-01-01 22:10:23
TIMESTAMPTZ Store date with timezone (e.g. last login) 2019-01-01 22:10:23+00
BYTEA Store binary data an image

There are many more data types. You can read about them here.

For a video explanation of the above table have a look at:

{% hyf-youtube src="https://www.youtube.com/watch?v=HnnPgdL0snI" %}

How to use SQL to Create, Read, Update and Delete (CRUD)

With the knowledge of all the data types, you can now create tables that contain the data with these data types.

Tables contain columns and columns have data types. For example, in a column with names of students, you cannot have numbers.

  • SQL provides a CREATE TABLE statement that creates a table with columns. You can choose the table name, column names but you have to choose the pre-defined data types supported by PostgreSQL. For example, a column Registration number cannot have the data type number. It must use INTEGER because it represents the numeric datatype.

  • SQL provides SELECT statement which reads (columns and rows) from a table with or without filtration.

  • SQL provides UPDATE statement which changes the contents of (columns and rows of) a table.

  • SQL provides DELETE statement which can delete rows of tables. In order to delete columns, you need to use ALTER and DROP statements.

Check out the following to learn more about how to apply SQL:

{% hyf-youtube src="https://www.youtube.com/watch?v=WZngwCbfakc" %}

{% hyf-youtube src="https://www.youtube.com/watch?v=bEtnYWuo2Bw" %}

{% hyf-youtube src="https://www.youtube.com/watch?v=OlT3FispsMU" %}