Though it's often useful to separate data into discrete tables for performance and consistency purposes, you often need to consult data from multiple tables to answer certain requests. Joining tables is a way of combining the data from various tables by matching each record based on common field values. Table of Contents There are a few different types of joins, which offer various ways of combining table records. In this article, we'll cover how MySQL implements joins and discuss the scenarios in which each is most useful. What are joins?In short, joins are a way of displaying data from multiple tables. They do this by stitching together records from different sources based on matching values in certain columns. Each resulting row consists of a record from the first table combined with a row from the second table, based on one or more columns in each table having the same value. The basic syntax of a join looks like this:
In a join, each resulting row is constructed by including all of the columns of the first table followed by all of the columns from the second table. The Multiple rows may be constructed from the original tables if the values in the columns used for comparison are not unique. For example, imagine you have a column being compared from the first table that has two records with a value of "red". Matched with this is a column from the second table that has three rows with that value. The join will produce six different rows for that value representing the various combinations that can be achieved. The type of join and the join conditions determine how each row that is displayed is constructed. This impacts what happens to the rows from each table that do and do not have a match on the join condition. For the sake of convenience, many joins match the primary key on one table with an associated foreign key on the second table. Although primary and foreign keys are only used by the database system to maintain consistency guarantees, their relationship often makes them a good candidate for join conditions. Different types of joinsVarious types of joins are available, each of which will potentially produce different results. Understanding how each type is constructed will help you determine which is appropriate for different scenarios. Inner and Cross joinsThe default join is called an inner join. In MySQL, this can be specified using either Here is a typical example demonstrating the syntax of an inner join:
An inner join is the most restrictive type of join because it only displays rows created by combining rows from each table. Any rows in the constituent tables that did not have a matching counterpart in the other table are removed from the results. For example, if the first table has a value of "blue" in the comparison column, and the second table has no record with that value, that row will be suppressed from the output. If you represent the results as a Venn diagram of the component tables, an inner join allows you to represent the overlapping area of the two circles. None of values that only existed in one of the tables are displayed. As mentioned above, MySQL also uses this format to produce cross joins. In MySQL, you can produce a cross join using an inner join without any match conditions. A cross join does not use any comparisons to determine whether the rows in each table match one another. Instead, results are constructed by simply adding each of the rows from the first table to each of the rows of the second table. This produces a Cartesian product of the rows in two or more tables. In effect, this style of join combines rows from each table unconditionally. So, if each table has three rows, the resulting table would have nine rows containing all of the columns from both tables. For example, if you have a
table called
Left joinA left join is a join that shows all of the records found in an inner join, plus all of the unmatched rows from the first table. In MYSQL, this can be specified as a The basic syntax of a left join follows this pattern:
A left
join is constructed by first performing an inner join to construct rows from all of the matching records in both tables. Afterwards, the unmatched records from the first table are also included. Since each row in a join includes the columns of both tables, the unmatched columns use If you represent the results as a Venn diagram of the component tables, a left join allows you to represent the entire left circle. The parts of the left circle represented by the intersection between the two circles will have additional data supplemented by the right table. Right joinA right join is a join that shows all of the records found in an inner join, plus all of the unmatched rows from the second table. In MySQL, this can be specified as a The basic syntax of a right join follows this pattern:
A right join is constructed by first
performing an inner join to construct rows from all of the matching records in both tables. Afterwards, the unmatched records from the second table are also included. Since each row in a join includes the columns of both tables, the unmatched columns use If you represent the results as a Venn diagram of the component tables, a right join allows you to represent the entire right circle. The parts of the right circle represented by the intersection between the two circles will have additional data supplemented by the left table. For portability reasons, MySQL recommends you use left joins instead of right joins where possible. Full joinA full join is a join that shows all of the records found in an inner join, plus all of the unmatched rows from both component tables. MySQL does not natively implement full joins, but we can emulate the behavior using a few tricks. To replicate the results of a full outer join, we will perform a left join to all of the results that are shared by both tables and all of the unmatched rows from the left table. Then we will use the The basic syntax of a full join follows this pattern:
Since each row in a join
includes the columns of both tables, the unmatched columns use If you represent the results as a Venn diagram of the component tables, a full join allows you to represent both of the component circles entirely. The intersection of the two circles will have values supplied by each of the component tables. The parts of the circles outside of the overlapping area will have the values from the table they belong to,
using Self joinA self join is any join that combines the rows of a table with itself. It may not be immediately apparent how this could be useful, but it actually has many common applications. Often, tables describe entities that can fulfill multiple roles in relationship to one another. For instance, if you have a table of Since self joins reference the same table twice, table aliases are required to disambiguate the references. In the example above, for instance, you could join the two instances of the Here is another example, this time representing relationships between employees and managers:
Join conditionsWhen combining tables, the join condition determines how rows will be matched together to form the composite results. The basic premise is to define the columns in each table that must match for the join to occur on that row. The ON clauseThe most standard way of defining the conditions for table joins is with the The The basic syntax of the
Here, the rows from This means that both the
The USING clauseThe The general syntax of the
This join combines This same join could be expressed more verbosely using
While both of the above joins would
result in the same rows being constructed with the same data present, they would be displayed slightly different. While the The NATURAL clauseThe The general syntax of the
Assuming that
And this query using the
Like the While the Join conditions and the WHERE clauseJoin conditions share many characteristics with the comparisons used to filter rows of data using In order to understand the differences that will result, we have to take a look at the order in which MySQL processes different portions of a query. In this case, the predicates in the join condition are processed first to construct the virtual joined table in memory. After this stage, the expressions within
the As an example, suppose that we have two tables called Given the above requirements, we have two conditions that we care about. The way we express these conditions, however, will determine the results we receive. First, let's use both as the join conditions for a
The results could potentially look something like this:
MySQL arrived at this result by performing the following operations:
The outcome is that all of our joined rows match both of the conditions that we are looking for. However, the left join causes MySQL to also include any rows from the first table that did not satisfy the join condition. This results in "left over" rows that don't seem to follow the apparent intent of the query. If we move the second query (
This time, only three rows are displayed:
The order in which the comparisons are executed is the reason for these differences. This time, MySQL processes the query like this:
This time, even though we are using a left join, the Understanding the basic process that MySQL uses to execute your queries can help you avoid some easy-to-make but difficult-to-debug mistakes as you work with your data. ConclusionIn
this article, we discussed what joins are and how MySQL implements them as a way of combining records from multiple tables. We covered the different types of joins available and the way that different conditions like the As you get more familiar with joins, you'll be able to use them as a regular part of your toolkit to pull in data from various sources and stitch together pieces of information to create a more full picture. Joins help bring together the data that organization principles and performance considerations may separate. Learning how to effectively use joins can help you bring together data regardless of how it's organized in the system. RELATED ON PRISMA.IO Prisma allows you to define relations between models in the Prisma schema file. You can then use relation queries to work with data that spans multiple models. Prisma is an open-source database toolkit for Typescript and Node.js that aims to make app developers more productive and confident when working with databases. About the Author(s) Justin EllingwoodJustin has been writing about databases, Linux, infrastructure, and developer tools since 2013. He currently lives in Berlin with his wife and two rabbits. He doesn't usually have to write in the third person, which is a relief for all parties involved. Can you inner join 2 tables?You can use an INNER JOIN operation in any FROM clause. This is the most common type of join. Inner joins combine records from two tables whenever there are matching values in a field common to both tables. How do I join two inner joins in MySQL?In this case the two tables are joined using the relationship table1.id = table2.id . It is possible to use multiple join statements together to join more than one table at the same time. To do that you add a second INNER JOIN statement and a second ON statement to indicate the third table and the second relationship. How can I join two tables in MySQL?Ans: Joining two tables in SQL can be done in four major ways: Inner Join (returns rows with matching columns), Left Join (ALL records in the left table and matching records in the right table), Right Join (ALL records in the right table and matching records in the left table), and Union (removes duplicates). How use inner join in MySQL example?MySQL INNER JOIN Keyword. SELECT column_name(s) FROM table1. INNER JOIN table2. ON table1.column_name = table2.column_name;. Example. SELECT Orders.OrderID, Customers.CustomerName. FROM Orders. ... . Example. SELECT Orders.OrderID, Customers.CustomerName, Shippers.ShipperName. FROM ((Orders.. |