In our first example, we want to know the education level of the teacher for each student. It’s time for a couple of SQL queries! Example 1: SQL JOIN by Two Columns However, even with the data stored like this, we can join the tables as long as each table has a set of columns that uniquely identifies each record. Typically, the students table would include foreign keys like the teacher ID and the class ID instead of detailed information about the corresponding teachers and classes. You might notice our database is not perfectly organized. classes, where we have information about each class, such as the name of the kindergarten, the class, the graduation year, and the name of the classroom.teachers, where we have the name and the education level of each teacher.students, where we have information about each student, such as the name, the kindergarten he or she attended, the class, the graduation year, and the teacher.In our database, we have the following tables: Let’s imagine we run a network of kindergartens. Let’s see some examples to understand how this works in practice. The explanations are based on real-world examples that resemble problems you'll meet daily. It contains over 90 exercises that cover different JOIN topics: joining multiple tables, joining by multiple columns, different JOIN types ( LEFT JOIN, RIGHT JOIN, FULL JOIN), or joining table with itself. The best way to practice SQL JOINs is our interactive SQL JOINs course. In situations like these, you may need to use multiple columns to join tables – e.g., the first and the last names, or the order number and the year if the order numbering restarts each year. Or the tables you want to join may not have just one common column to use for joining. For example, you may encounter cases in which there is no one column in the table that uniquely identifies the rows. Joining tables by just one column does not work in some scenarios. In this article, I’ll discuss why you would want to join tables by multiple columns and how to do this in SQL. However, it is also often the case that you need to join tables by two or more columns. Most often, you’ll be joining tables based on a primary key from one table and a foreign key from another table. categoryName from the categories table.The SQL JOIN is an important tool for combining information from several tables.productID, productName from the products table.We need to query the following data from both tables: The link between the two tables is the categoryid column. Therefore, there is a many-to-one relationship between the rows in the categories table and rows in the products table. One product belongs to one and only one category.The following picture illustrates the database diagram. In this example, we will use the products and categories tables in the sample database. SQL INNER JOIN examples SQL INNER JOIN – querying data from two tables example Otherwise, it examines next row in the table_1, and this process continues until all the rows in the table_1 are examined.įor joining more than two tables, the same logic was applied. If the corresponding row found, the query returns a row that contains data from both tables. For each row in the table_1, the query find the corresponding row in the table_2 that meet the join condition.The table_1 and table_2 are called joined-tables.Let’s examine the syntax above in greater detail: INNER JOIN table_2 ON join_condition Code language: SQL (Structured Query Language) ( sql )
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