Data Query Language (DQL) Queries

Data Query Language (DQL) queries in SQL are used to retrieve data from a relational database. The primary SQL statement associated with DQL is the SELECT statement, which allows you to specify what data you want to retrieve and how you want it to be presented. Here are the key aspects of DQL queries:

SELECT Statement: The SELECT statement is the core of DQL queries and is used to retrieve data from one or more tables in a database.
Columns: You specify the columns you want to retrieve data from in the SELECT statement. You can select specific columns or use the wildcard `` to select all columns.

Example:
```
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SELECT first_name, last_name FROM employees;
```
Tables: You specify the tables from which you want to retrieve data in the FROM clause of the SELECT statement.

Example:
```
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SELECT product_name, price FROM products;
```
Filtering Data: You can use the WHERE clause to filter the rows you retrieve based on specific conditions. This allows you to narrow down the data to only the records that meet certain criteria.

Example:
```
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SELECT order_id, customer_id, order_date FROM orders WHERE order_date >= '2023-01-01';
```
Sorting Data: You can use the ORDER BY clause to specify how the retrieved data should be sorted, either in ascending (ASC) or descending (DESC) order.

Example:
```
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SELECT product_name, price FROM products ORDER BY price DESC;
```
Aggregation: DQL queries often involve aggregate functions like SUM, AVG, COUNT, MIN, and MAX to perform calculations on groups of data.

Example:
```
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SELECT category_id, COUNT(*) AS product_count FROM products GROUP BY category_id;
```
Aliases: You can use aliases to assign temporary names to columns or expressions in the result set. This is helpful for renaming columns or making the output more readable.

Example:
```
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SELECT first_name AS "First Name", last_name AS "Last Name" FROM employees;
```
Joins: When you need to retrieve data from multiple tables, you can use JOIN clauses to combine data based on common columns between the tables.

Example:
```
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SELECT orders.order_id, customers.customer_name
FROM orders
JOIN customers ON orders.customer_id = customers.customer_id;
```
Subqueries: DQL queries can include subqueries (queries nested within other queries) to retrieve data conditionally or as part of calculations.

Example:
```
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SELECT product_name, price
FROM products
WHERE price > (SELECT AVG(price) FROM products);
```
Limiting Rows: You can limit the number of rows returned using the LIMIT clause. This is useful when you want to retrieve only a specified number of rows.

Example:
```
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SELECT product_name, price FROM products LIMIT 10;
```

DQL queries are fundamental in SQL and are used extensively in data retrieval, reporting, and analysis tasks. They provide the means to access and extract data from relational databases based on specific criteria and conditions.