SQL stands for Structured Query Language, and has been specifically designed for working with relational databases. This type of database is based on the concept of primary keys and the corresponding data stored within a table. Since databases today can handle billions of rows of data and process tons of queries at the same time, a language that helps to obtain necessary pieces of information within seconds is highly required.
One of the most interesting types of queries is the subquery. It is often used within large databases in order to find necessary data without creating multiple distinct queries or additional tables. It works very simply: when writing the main query, we include the subquery that is executed first, and then based on its results, the main query can run and produce the desired outcomes. For example, the query shown below first searches for the customer Bill Richardson, finds his city, then searches for orders.oID, customers.Name, products.name, and shops.name, where the month of the order is December, and the city is the same as the one Bill Richardson lives in.
Also, significant points worth considering in SQL are those pertinent to working with NULL values, as it is necessary to know how to handle these values. They are known to be of any type, containing nothing that can be compared to other values.
Such functions as ISNULL(), NULLIF(), and COALESCE() are used to address issues pertinent to NULLs.
The ISNULL() function is used to replace NULL values with some appropriate value.
The NULLIF() function has two parameters that are compared, and in the case where both of them are equal, NULL is returned. Alternatively, the first parameter will be returned.
COALESCE() in its turn returns the first non NULL values in the data set specified as a parameter of this function.
Regarding the first example, we can suppose that the price of the product (products.price) of NULL can be replaced with 10. So, if we want to calculate the overall price of all the products, the query will look the following way:
SELECT SUM(ISNULL(products.price, 10)) as Sum
Using the aforementioned functions, it is possible to deal with all kinds of values, as we can instantly replace NULL with some adequate value; or we can compare them with some other values and produce the appropriate result based on this comparison.
Another widely known type of query is related to finding several records of max, min, or simply the most recent ones. Let’s assume there is a table containing data about orders and products purchased along with data of the operation. And we need to find the three most recently performed purchases.
In this sample we have considered several widely used types of queries in order to get familiar with the basics of SQL, along with brief theoretical information about this language.
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