vimwiki/tech/sql.wiki

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= SQL =
Stuctered Query Language is a language that describes a method of fetching and
describing the relationship between different types of data
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== Filtering data ==
To filter data we by some specifiers we use the `WHERE` clause. The where
clause can filter based on the default comparison operators. It can also do
string comparisons.
This system also works with dates, where all dates are in format `YYYY-MM-DD`.
=== IN ===
To see if an attribute is in a set we can use the `IN` keyword in conjunction
with a `WHERE` statement. For example,
`SELECT * FROM holidays WHERE Country IN ('spain', 'portugaul', 'USA');`
=== NOT ===
The `NOT` keyword can be placed before a conditional to specify that we want
everything that does NOT mean that condition.
=== LIKE ===
`LIKE` lets us filter on patterns. We can use the character `%` to specify a
wildcard, for any number of characters. We can use the character `_` to give an
exact number of wildcard characters we are looking for. For example,
`SELECT * FROM Bands WHERE BandName LIKE '%light%';`
will return all bands with the string 'light' in their name.
=== DISTINCT ===
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`DISTINCT` filters results by only returning unique values of a column.
Therefore, we can only be used to return a single column.
{{{
SELECT DISTINCT nationality FROM users;
}}}
== Presentation ==
=== Order by ===
`ORDER BY` allows us to descide how the query will be shown. For example,
{{{
SELECT Firstname, Lastname
FROM users
ORDER BY Lastname [SORT];
}}}
where `[SORT]` is
* DESC
* descending order
* ASC
* ascending order
=== CASE ===
Case is a statement block where we can choose exactly what text is returned
based on values in the database.
{{{
SELECT Name,
CASE
WHEN Country = 'USA' THEN 'North America'
WHEN Country = 'UK' THEN 'Europe'
ELSE 'Wolrd'
END
FROM Users;
}}}
=== LIMIT ===
`LIMIT` allows us to retstrict the number or rows returned.
`SELECT * FROM users ORDER BY name DESC LIMIT 1`
=== COUNT ===
If we want just the number of rows, we can use count.
`SELECT COUNT(*) FROM users WHERE dob > '2000-01-01'`
=== math functions ===
If we want to sum the values in ONE column we can use sum
`SELECT SUM(cash_balance) FROM users`
If we want to find an average,
`SELECT AVG(price) FROM products`
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This same principle applies for
* `MAX`
* `MIN`
=== GROUP BY ===
`GROUP BY` allows us to combine with the above math and `COUNT` functions by
grouping totals based upon some other row.
`SELECT Team, COUNT(*) FROM Players GROUP BY Team`
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=== JOIN ===
`JOIN` allows two tables to be returned together, where they are 'joined' upon
a shared field.
{{{
SELECT Orders.ID, Customers.Name, Orders.Name
FROM Orders
INNER JOIN Customers ON Orders.CustomerId=Customers.Id
}}}
There are four types of joins
* `INNER JOIN` returns all records that having matching records in both tables
* `LEFT OUTER JOIN` returns all records from the left (Orders in above example)
table, and the matched records from the right table
* `RIGHT OUTER JOIN` returns all records from the right (Customers in above
example) table, and the matched records from the left table
* `FULL OUTER JOIN` returns all records when there is a match in either the
left or right table
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== Views ==
A view can be created with the following command
{{{
CREATE VIEW [view_name] AS
SELECT column1, column2, ...
FROM table
WHERE condition;
}}}
For example, this view returns all products witha price higher than the average
price of the table
{{{
CREATE VIEW [Products above average price] AS
SELECT Name, Price
FROM Products
WHERE Price > (SELECT AVG(Price) FROM Products)
SELECT * FROM [Prodcuts above average price]
}}}
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Views *can* be inserted into, you simply must specify the columns being
inserted into.
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== Acess control commands ==
Two commands for managing access rights
* grant
* used to grant one or more access rights or can be used to assign a user to
a role
* revoke
* revokes the acess rights
Typical access rights include
* select
* insert update
* delete
* references
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== Also see ==
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[[databases]]
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[[index]]