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5.3 KiB
Markdown
146 lines
5.3 KiB
Markdown
# Basic Syntax
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In this chapter, we will go over the basic SQL syntax.
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SQL statements are basically the 'commands' that you run against a specific database. Through the SQL statements, you are telling MySQL what you want it to do, for example, if you wanted to get the `username` of all of your users stored in the `users` table, you would run the following SQL statement:
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```sql
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SELECT username FROM users;
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```
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Rundown of the statement:
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* `SELECT`: First, we specify the `SELECT` keyword, which indicates that we want to select some data from the database. Other popular keywords are: `INSERT`, `UPDATE` and `DELETE`.
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* `username`: Then we specify which column we want to select.
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* `FROM users`: After that, we specify the table that we want to select the data from using the `FROM` keyword.
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* The semicolon `;` is highly recommended to put at the end. Standard SQL syntax requires it, but some "Database Management Systems' (DBMS)" are tolerant about it, but it's not worth the risk.
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If you run the above statement, you will get no results as the new `users` table that we've just created is empty.
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> As a good practice, all SQL keywords should be with uppercase, however, it would work just fine if you use lower case as well.
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Let's go ahead and cover the basic operations next.
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## INSERT
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To add data to your database, you would use the `INSERT` statement.
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Let's use the table that we created in the last chapter and insert 1 user into our `users` table:
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```sql
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INSERT INTO users (username, email, active)
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VALUES ('bobby', 'bobby@bobbyiliev.com', true);
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```
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Rundown of the insert statement:
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* `INSERT INTO`: first, we specify the `INSERT INTO` keyword, which tells MySQL that we want to insert data a table.
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* `users (username, email, active)`: then, we specify the table name `users` and the columns that we want to insert data into.
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* `VALUES`: then, we specify the values that we want to insert in. The order of attributes is the same as in `users (...)`.
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## SELECT
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Once we've inserted that user, let's go ahead and retrieve the information.
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To retrieve information from your database, you could use the `SELECT` statement:
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```sql
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SELECT * FROM users;
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```
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Output:
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```
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+----+----------+-------+----------+--------+---------------+
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| id | username | about | birthday | active | email |
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+----+----------+-------+----------+--------+---------------+
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| 1 | bobby | NULL | NULL | 1 | bobby@b...com |
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+----+----------+-------+----------+--------+---------------+
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```
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We specify `*` right after the `SELECT` keyword, this means that we want to get all of the columns from the `users` table.
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If we wanted to retrieve only the `username` and the `email` columns instead, we would change the statement to:
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```sql
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SELECT username, email FROM users;
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```
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This will return all of the users, but as of the time being we have only 1:
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```
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+----------+----------------------+
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| username | email |
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+----------+----------------------+
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| bobby | bobby@bobbyiliev.com |
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+----------+----------------------+
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```
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## UPDATE
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In order to modify data in your database, you could use the `UPDATE` statement.
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The syntax would look like this:
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```sql
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UPDATE users SET username='bobbyiliev' WHERE id=1;
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```
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Rundown of the statement:
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* `UPDATE users`: First, we specify the `UPDATE` keyword followed by the table that we want to update.
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* `SET username='bobbyiliev'`: Then we specify the columns that we want to update and the new value that we want to set.
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* `WHERE id=1`: Finally, by using the `WHERE` clause, we specify which user should be updated. In our case it is the user with ID 1.
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> NOTE: If we don't specify a `WHERE` clause, all of the entries inside the `users` table would be updated, and all users would have the `username` set to `bobbyiliev`. You need to be careful when you use the `UPDATE` statement without a `WHERE` clause, as every single row will be updated.
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We are going to cover `WHERE` more in-depth in the next few chapters.
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## DELETE
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As the name suggests, the `DELETE` statement would remove data from your database.
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The syntax is as follows:
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```sql
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DELETE FROM users WHERE id=1;
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```
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Similar to the `UPDATE` statement, if you don't specify a `WHERE` clause, all of the entries from the table will be affected, meaning that all of your users will be deleted.
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## Comments
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In case that you are writing a larger SQL script, it might be helpful to add some comments so that later on, when you come back to the script, you would know what each line does.
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As with all programming languages, you can add comments in SQL as well.
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There are two types of comments:
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* Inline comments:
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To do so, you just need to add `--` before the text that you want to comment out:
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```sql
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SELECT * FROM users; -- Get all users
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```
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* Multiple-line comments:
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Similar to some other programming languages in order to comment multiple lines, you could wrap the text in `/*` `*/` as follows:
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```sql
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/*
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Get all of the users
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from your database
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*/
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SELECT * FROM users;
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```
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You could write that in a `.sql` file and then run it later on, or execute the few lines directly.
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## Conclusion
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Those were some of the most common basic SQL statements.
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In the next chapters, we are going to go over each of the above statements more in-depth.
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