SQL queries can join tables together from "summary" of SQL and Relational Theory by C. Date
In the relational model, data is organized into tables, where each table represents an entity or a relationship between entities. Tables can be connected through common attributes, allowing us to retrieve information from multiple tables using SQL queries. This process is known as joining tables together. When joining tables in SQL, we specify the columns that we want to use as the basis for the join. These columns must have the same data type and contain related information. By matching the values in these columns, we can create a result set that combines data from both tables. There are different types of joins that can be used in SQL queries, such as INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN. Each type of join has a specific purpose and determines how the rows from the tables are combined in the result set. In an INNER JOIN, only the rows with matching values in both tables are included in the result set. This type of join is useful for retrieving data that has a relationship between the tables. On the other hand, a LEFT JOIN includes all the rows from the left table, even if there are no matching values in the right table. Similarly, a RIGHT JOIN includes all the rows from the right table. A FULL JOIN combines the results of both the LEFT JOIN and the RIGHT JOIN, ensuring that all rows from both tables are included in the result set. This type of join is useful when we want to see all the data from both tables, regardless of whether there is a match. By joining tables in SQL queries, we can create a comprehensive view of the data by combining information from multiple tables. This allows us to retrieve relevant data and perform complex analyses that involve relationships between entities. Joins are a fundamental concept in SQL and are essential for querying relational databases effectively.
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