An SQL query is a set of instructions that can be used to retrieve data from a database.
In this tutorial, you will learn how to write a SQL query to retrieve the data you need from a SQLite database. You will learn:
– How to connect to a database
– What is a SELECT statement and WHERE clause
– JOIN and how to GROUP BY a table
– A HAVING clause to filter the results
– The ORDER BY clause
– An INSERT INTO statement to add new data to a table
## What is SQL?
SQL stands for Structured Query Language and is a language used to communicate with databases. It is a declarative language, meaning that you tell the database what you want to retrieve, rather than telling it how to retrieve it. This means that you do not need to worry about the details of how the database retrieves the data. You simply tell it what data you want, and it will figure out the best way to get it for you.
## Connect to a Database
Before you can write any SQL queries, you must first connect to the database. This is done using the `sqlite3` command-line tool.
You will be prompted for a password, which you will need to enter. If you don’t know the password, you can use the `–password` option to specify a password for the current database.
Once you have connected to your database, you are ready to start writing SQL queries!
## Connecting to multiple databases
If you have more than one database on your computer, it is possible to connect multiple databases at the same time. To do this, simply add the database name to the end of the connection string.
This will open a new connection to the specified database, and you will be able to write SQL queries against that database. To close the connection, simply type `quit` at the command prompt.
To connect to all the databases on your system.
It is also possible to use wildcards when specifying the name of the database, so that you can specify multiple databases with a single command.
The `*` character is a wildcard that matches any string of characters. You can use this wildcard to specify any number of database names, so long as they are separated by a space.
## SELECT Statement
A SELECT statement is the most common type of SQL query. A SELECT statement retrieves data from one or more tables in a database and returns it to the user. The data is returned in the form of a table, which is a list of rows and columns. Each row is a record, and each column is a field in that record. For each record, the columns are listed in the order in which they appear in the table, and the values in each column are listed next to the column name.
We can write a SELECT statement that will return the data from this table.
The SELECT statement has two parts. The first part is the `SELECT` keyword, which tells the database that you are going to retrieve a table of data. The second part is a `FROM` clause, which specifies the table from which the data is going to be retrieved. In the example above, the table is named `people`, and the data will be retrieved from the rows in that table. If the table does not have any rows, the SELECT statement will return an empty table.