Structured Query Language (SQL)
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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.
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.
SQL SELECT queries
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.
SELECT column, another_column, …
FROM mytable;
If we want to retrieve absolutely all the columns of data from a table, we can then use the asterisk (*) shorthand in place of listing all the column names individually.
SELECT *
FROM mytable;
Queries with constraints
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.
SELECT column, another_column, …
FROM mytable
WHERE condition
AND/OR another_condition
AND/OR …;