Introduction to SQL

What is SQL?

SQL, or Structured Query Language, is a language designed to allow both technical and non-technical users query, manipulate, and transform data from a relational database. And due to its simplicity, SQL databases provide safe and scalable storage for millions of websites and mobile applications.

There are many popular SQL databases including SQLite, MySQL, Postgres, Oracle and Microsoft SQL Server. All of them support the common SQL language standard, which is what this site will be teaching, but each implementation can differ in the additional features and storage types it supports.

Relational databases

Before learning the SQL syntax, it’s important to have a model for what a relational database actually is. A relational database represents a collection of related (two-dimensional) tables. Each of the tables are similar to an Excel spreadsheet, with a fixed number of named columns (the attributes or properties of the table) and any number of rows of data.

SELECT queries 101

To retrieve data from a SQL database, we need to write SELECT statements, which are often colloquially refered to as queries. A query in itself is just a statement which declares what data we are looking for, where to find it in the database, and optionally, how to transform it before it is returned. It has a specific syntax though.

Queries with constraints (Pt. 1)

Now we know how to select for specific columns of data from a table, but if you had a table with a hundred million rows of data, reading through all the rows would be inefficient and perhaps even impossible.

In order to filter certain results from being returned, we need to use a WHERE clause in the query. The clause is applied to each row of data by checking specific column values to determine whether it should be included in the results or not.

Queries with constraints (Pt. 2)

When writing WHERE clauses with columns containing text data, SQL supports a number of useful operators to do things like case-insensitive string comparison and wildcard pattern matching.

Filtering and sorting Query results

Even though the data in a database may be unique, the results of any particular query may not be – take our Movies table for example, many different movies can be released the same year. In such cases, SQL provides a convenient way to discard rows that have a duplicate column value by using the DISTINCT keyword.

Inserting rows

What is a Schema?

We previously described a table in a database as a two-dimensional set of rows and columns, with the columns being the properties and the rows being instances of the entity in the table. In SQL, the database schema is what describes the structure of each table, and the datatypes that each column of the table can contain.

Inserting new data

When inserting data into a database, we need to use an INSERT statement, which declares which table to write into, the columns of data that we are filling, and one or more rows of data to insert. In general, each row of data you insert should contain values for every corresponding column in the table. You can insert multiple rows at a time by just listing them sequentially.

Updating rows

In addition to adding new data, a common task is to update existing data, which can be done using an UPDATE statement. Similar to the INSERT statement, you have to specify exactly which table, columns, and rows to update. In addition, the data you are updating has to match the data type of the columns in the table schema.

Deleting rows

When you need to delete data from a table in the database, you can use a DELETE statement, which describes the table to act on, and the rows of the table to delete through the WHERE clause.

Creating tables

When you have new entities and relationships to store in your database, you can create a new database table using the CREATE TABLE statement.

Altering tables

As your data changes over time, SQL provides a way for you to update your corresponding tables and database schemas by using the ALTER TABLE statement to add, remove, or modify columns and table constraints.

Dropping tables

In some rare cases, you may want to remove an entire table including all of its data and metadata, and to do so, you can use the DROP TABLE statement, which differs from the DELETE statement in that it also removes the table schema from the database entirely.