Uso práctico del WHERE en SQL para filtrar datos con precisión

Filtering information effectively and accurately is essential in SQL, especially when working with large volumes of data. Proper use of the WHERE command can make the difference between querying relevant information or getting lost in excessive results. This content shows you how to use WHERE in your SQL queries to get only the data you really need.

What is WHERE and how does it work in SQL?

The WHERE clause in SQL allows you to specify conditions to select specific records in a database. Each record or tuple will be evaluated individually, returning only those that meet the conditions stated in the WHERE.

The expressions used in the WHERE clause result in a boolean value: true or false. SQL will check each record according to these conditions, evaluating them accurately tuple by tuple.

How is a WHERE query structured?

The basic structure you can use is:

SELECT column1, column2FROM tableWHERE condition;

This clause can include logical operations such as AND, OR, as well as functions on strings, numbers and dates.

What operators can you use with the WHERE?

• Comparison operators: =, >, <, >=, <=, !=.
• Logical operators: AND, OR, NOT.
• Special operators: LIKE, BETWEEN, IN, RAND().

How to use the LIKE command to find specific patterns?

To search for specific patterns in text strings, MySQL provides the LIKE function accompanied by wildcards. Useful examples:

SELECT  * FROM Clients WHERE Name LIKE 'Mr.%';

In this case, SQL will fetch all people whose name starts with "Mr.".

If you want to search for names with specific endings:

SELECT  * FROM Clients WHERE Name LIKE '%III' OR Name LIKE '%IV';

You can also search for a specific word to appear anywhere in the name:

SELECT  * FROM Clients WHERE Name LIKE '%Gibson%';

How to use BETWEEN on numeric ranges and dates?

The BETWEEN operator allows you to specify both numeric and date ranges for filtering:

SELECT  * FROM Products WHERE Price  BETWEEN 500 AND 700;

For dates:

SELECT  * FROM bill_productsWHERE date_added  BETWEEN '2024-09-24' AND '2024-09-30';

Can you use advanced logical operators to obtain complex selections?

The basic structure for combining conditions is using AND and OR, achieving more specific filters:

SELECT  * FROM bill_productsWHERE date_added  BETWEEN '2024-09-24' AND '2024-09-30'AND product_id IN (825, 500, 1986);

This allows for combinations of filters that will increase the accuracy and usefulness of your queries.

How to change the query display for long records?

To improve the display for long records, replace the semicolon that ends the query with a semicolon, which causes the results to be presented vertically:

SELECT  * FROM clients LIMIT 1\G

This makes it much easier to read when tables have many columns or data.

Remember that practicing and experimenting directly in a database will help you internalize these concepts. We invite you to try the files provided to become more familiar with accurate SQL queries.