SQL Join Tables

10 Key excerpts on "SQL Join Tables"

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  eBook - ePub

  Querying SQL Server

  Run T-SQL operations, data extraction, data manipulation, and custom queries to deliver simplified analytics (English Edition)

  • Adam Aspin(Author)
  • 2022(Publication Date)
  • BPB Publications

    (Publisher)

  Figure 2.3 illustrates what joining tables looks like.

  Figure 2.3: Representing table joins visually

  Conceptually, joining pairs of tables means knowing which fields can be used to create the “bridge” between the two tables. Once again, this could mean talking to the people who designed the database or delving into any documentation that you may have been given. Sometimes you can easily guess which fields can be used as joins, such as when the two fields have the same name or when the data is clearly the same in both tables. At other times it can be really difficult to guess the join fields just by looking at the tables. For the moment, I will explain how the sample tables are joined each time to avoid you having to guess.

  Practically, this code snippet takes three keywords that you already know well (SELECT, FROM, and ORDER BY) and extends the FROM clause with two new keywords: INNER JOIN and ON . These keywords allow you to create links between tables so that you can display fields from both tables at once. Linking (or joining) two tables in SQL consists of the following steps:

  1. Enter the name of the first table to join after the FROM keyword.
  2. Add the INNER JOIN keyword after the table name.
  3. Enter the name of the second table to join.
  4. Add the ON keyword.
  5. Enter the names of the two fields that are the link between the two tables, separated by the equal (=) sign.

  Each of the fields used to link the tables must be preceded by the name of the table that contains the field followed by a period. This enables SQL Server to identify which field can be found in which table when creating the link.

  Tricks and Traps

  As joining tables is a core concept, you should note these key points from the start:

  • The fields that you use to join the two tables may—or may not—have the same name in both tables. If the two field names are different, then you can just enter them “as is,” separated by an equal sign without adding the table name. If, however, the two fields have the same name, then you need to precede each field name by the table name and a period. This is because SQL Server gets confused if there is no way of uniquely identifying the field name—almost as if it doesn’t know which one to take to finish the join. Adding the table name and a period like this allows SQL to trace the field name back to its source table and consequently identify the correct field to use in the join.

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  eBook - ePub

  Querying MySQL

  Make your MySQL database analytics accessible with SQL operations, data extraction, and custom queries (English Edition)

  • Adam Aspin(Author)
  • 2022(Publication Date)
  • BPB Publications

    (Publisher)

  Figure 2.3 illustrates what joining tables looks like.

  Figure 2.3: Representing table joins visually

  Conceptually, joining pairs of tables means knowing which fields can be used to create the “bridge” between the two tables. Once again, this could mean talking to the people who designed the database or delving into any documentation that you may have been given. Sometimes you can easily guess which fields can be used as joins, such as when the two fields have the same name or when the data is clearly the same in both tables. At other times it can be really difficult to guess the join columns just by looking at the tables. For the moment, I will explain how the sample tables are joined each time to avoid you having to guess.

  Practically, this code snippet takes three keywords that you already know well (SELECT, FROM, and ORDER BY) and extends the FROM clause with two new keywords: JOIN and USING. These keywords allow you to create links between tables so that you can display columns from both tables at once. Linking (or joining) two tables in SQL consists of the following steps:

  First	Enter the name of the first table to join after the FROM keyword.
  Second	Add the JOIN keyword after the table name.
  Third	Enter the name of the second table to join to the first.
  Then	Add the USING keyword and a left parenthesis.
  Finally	Enter the name of the field that is the link between the two tables followed by a right parenthesis.

  Tricks and Traps

  As joining tables is a core concept, you should note these key points from the start:

  • This particular join technique will only work when the two tables have a field with the same name in both tables that is used to join the tables. If this is not the case you will need to apply the technique described in the following section.
  • The order in which you enter the tables in the FROM clause is unimportant. You could write the FROM clause in this example as follows without altering the result: stock JOIN model

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  Querying MariaDB

  Use SQL Operations, Data Extraction, and Custom Queries to Make your MariaDB Database Analytics more Accessible (English Edition)

  • Adam Aspin(Author)
  • 2022(Publication Date)
  • BPB Publications

    (Publisher)

  Figure 2-3 illustrates what joining tables looks like.

  Figure 2-3: Representing table joins visually

  Conceptually, joining pairs of tables means knowing which fields can be used to create the “bridge” between the two tables. Once again, this could mean talking to the people who designed the database or delving into any documentation that you may have been given. Sometimes you can easily guess which fields can be used as joins, such as when the two fields have the same name or when the data is clearly the same in both tables. At other times it can be really difficult to guess the join columns just by looking at the tables. For the moment, I will explain how the sample tables are joined each time to avoid you having to guess.

  Practically, this code snippet takes three keywords that you already know well (SELECT, FROM, and ORDER BY) and extends the FROM clause with two new keywords: JOIN and USING. These keywords allow you to create links between tables so that you can display columns from both tables at once. Linking (or joining) two tables in SQL consists of the following steps:

  First	Enter the name of the first table to join after the FROM keyword.
  Second	Add the JOIN keyword after the table name.
  Third	Enter the name of the second table to join to the first.
  Then	Add the USING keyword and a left parenthesis.
  Finally	Enter the name of the field that is the link between the two tables followed by a right parenthesis.

  Tricks and Traps

  As joining tables is a core concept, you should note these key points from the start:

  • This particular join technique will only work when the two tables have a field with the same name in both tables that is used to join the tables. If this is not the case you will need to apply the technique described in the following section.
  • The order in which you enter the tables in the FROM clause is unimportant. You could write the FROM clause in this example as follows without altering the result:   stock JOIN model

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  eBook - PDF

  SQL Clearly Explained

  • Jan L. Harrington(Author)
  • 2003(Publication Date)
  • Morgan Kaufmann

    (Publisher)

  Retrieving Data from More Than One Table: Joins As you read in Chapter 3, logical relationships between entities in a relational database are represented by matching primary and for- eign key values. Given that there are no permanent connections be- tween tables stored in the database, a DBMS must provide some way for users to match primary and foreign key values when need- ed. This capability is provided by the relational algebra join opera- tion, which combines two tables based on a specified relationship between data they contain. In this chapter you will first learn about how joins work. Then you will be introduced to the syntax for including a join in a SQL query. Throughout this chapter you will read about the impact joins have on database performance. At the end you will see how subqueries (SELECTS within SELECTS) can be used to avoid joins, and in some 65 66 RETRIEVING DATA FROM MORE THAN ONE TABLE: JOINS cases, significantly decrease the time it takes for a DBMS to com- plete a query. The Relational Algebra Join Operation The relational algebra join creates one result table from two source tables by pasting a row from one source table onto the end of a row from the other source table. A DBMS uses a relationship between data in the two source tables to determine which rows should be combined. A Non-Database Example To help you understand how a join works, we will begin with an ex- ample that has absolutely nothing to do with relations. Assume that you have been given the task of creating manufacturing part assem- blies by connecting two individual parts. The parts are classified as either A parts or B parts. There are many types of A parts (A1 through An, where n is the total number of types of A parts) and many types of B parts (B1 through Bn). Each B is to be matched to the A part with the same number; conversely, an A part is to be matched to a B part with the same number.

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  eBook - PDF

  Oracle SQL

  Jumpstart with Examples

  • Gavin JT Powell, Carol McCullough-Dieter(Authors)
  • 2004(Publication Date)
  • Digital Press

    (Publisher)

  For the purposes of this Oracle SQL book, these facts will suffice. 1 Now let’s look at different types of joins you are able to build in Oracle. 10.2 Types of Joins Let’s look at the available types of joins and what exactly they do: Cross-join or Cartesian product. Merges all data selected from both tables into a single result set. Figure 10.2 ANSI Join Syntax. 208 10.2 Types of Joins Inner join. Combines rows from both tables using matching column names and column values. The result set includes only rows that match. Outer join. Selects rows from both tables as with an inner join but including rows from one or both tables that do not have matching rows in the other table. Missing values are replaced with null values. Left outer join. All rows from the left table plus all matching rows from the right table. Column values from the right table are replaced with null values when the matching right-side row does not exist in the left-side table. Right outer join. All rows from the table on the right plus match-ing rows from the left table, the opposite of the left outer join. Full outer join. All rows from both tables, with null values replacing missing values. Self-join. This joins a table to itself. Equi-joins, anti-joins, and range joins. An equi-join combines table data based on equality (=), an anti-join matches data based on inequality (!=, <> or NOT), and a range join compares data using a range of values (<, > or BETWEEN). Mutable and complex joins. A mutable join is a join of more than two tables. A complex join is a mutable join with added filtering. This is a lot of technical jargon. Let’s explain what joins are using simple mathematical set theory. Relational database theory is based on set theory. Do you remember learning set theory at school? A direct correlation can be made between simple set theory and relational database joins. Figure 10.3 shows two completely unrelated sets, Set A and Set B.

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  No longer available |Learn more

  SQL Pocket Primer

  • Oswald Campesato(Author)
  • 2022(Publication Date)
  • Mercury Learning and Information

    (Publisher)

  If you are not convinced of the preceding statement, consider this scenario: you have great performance in your SQL statements, but you aren’t sure if all the data is correct. If you have mission critical data that requires 100% data integrity, then data integrity has a higher priority than optimal performance.

  Fortunately, performance issues can sometimes be addressed by performing the appropriate denormalization of relevant tables. Note that this will involve rewriting the SQL statements that perform a JOIN of the normalized tables so that the new SQL statements query the single denormalized database table.

  Types of SQL JOIN Statements

  The JOIN keyword enables you to define various types of SQL statements that have slightly different semantics:

  • INNER JOIN
  • LEFT OUTER JOIN
  • RIGHT OUTER JOIN
  • CROSS JOIN
  • SELF-JOIN

  Let’s suppose that table A has some (but not all) corresponding rows in table B , and that table B has some (but not all) corresponding rows in table A . Moreover, let’s also assume that a JOIN statement specifies table A first and then table B .

  An INNER JOIN returns all rows from table A that have non-empty matching rows in another table.

  A LEFT JOIN returns all rows from left-side table A and either matching rows from the right-side table B or NULL if no matching rows in right-side table B .

  A RIGHT JOIN returns all rows from right-side table B and either matching rows from the left-side table A or NULL if no matching rows in table A .

  A CROSS JOIN is a Cartesian or “full” product of rows from left-side table A and right-side table B .

  A SELF JOIN joins a table to itself. A common use-case involves an employees table that contains a manager attribute for each employee. Given an employee in this table, find the value in the manager attribute for that employee, and then search the employees table a second time using the manager attribute.

  This sequence of steps can be repeated until the top-most employee does not have a manager (such as the CEO). Given an employee, the preceding sequence produces the management hierarchy from the employee to the topmost employee in a company (defined in the table).

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  eBook - PDF

  SQL: 1999

  Understanding Relational Language Components

  • Jim Melton, Alan R. Simon(Authors)
  • 2001(Publication Date)
  • Morgan Kaufmann

    (Publisher)

  We'll go over each piece as needed, and we'll do so methodically. Let's now take a look at each of these join operations, in turn. 230 Chapter 8 Working with Multiple Tables: The Relational Operators 8.3.1 Classic, Comma-Separated Joins So far, the SELECT statements we have examined have always included a single table name following the keyword FROM, as in SELECT column-I ist FROM si ngl e-tabl e-name You can, however, specify multiple table names after the FROM, which causes SOL to perform a relational join operation, as in SELECT table1.*, table2.* FROM table1, table2 ; (In spite of our using it in a few examples here and there, we strongly recom- mend that you code * in a select listmmeaning all columns of the table---only when you're using interactive SQL, but never in an application program.) The above SQL statement causes a virtual table to be produced that includes the Cartesian product (i.e., the cross-product) of the source tables, as illustrated in Result 8-1 when the music titles and current distributor costs tables are joined together in this manner. The resulting table is basically meaningless, since the cross-product of the join causes each row from the first table to be matched with each possible row from the second table. Result 8-1 Sample Old-StyleJoin with Cartesian Product Result MUSIC_ MUSIC_ DISTRIBUTOR_ OUR_ OUR_CASSETTE_ ID TITLE ARTIST...

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  eBook - ePub

  Querying Databricks with Spark SQL

  Leverage SQL to query and analyze Big Data for insights (English Edition)

  • Adam Aspin(Author)
  • 2023(Publication Date)
  • BPB Publications

    (Publisher)

  HAPTER 11Using Advanced Table Joins Introduction

  Joining tables to produce the data you want to see is key to creating the analyses that drive your business. In this chapter, you will build on the knowledge you acquired in the previous chapter and further develop your SQL skills so that you can create more complex and powerful queries. This means learning further variations of the JOIN clause to help you shape the data, which is the output from the SQL queries you write.

  Structure In this chapter, we will cover the following topics:

  • Filtering data using inner joins
  • Filtering data using multiple tables join
  • Semi joins
  • Filtering data output using intermediate tables
  • Using left joins to return all the data in one table but not from the other table
  • Right joins to return all the data in one table but not from the other
  • Full joins to return all the data from both tables in a join
  • Intermediate table joins
  • Using multiple fields in joins
  • Joining a table to itself
  • Joining tables on ranges of values
  • Cross joins
  • Join concepts

  Objectives This chapter will introduce most of the join types that are available in Spark SQL. These include the following:

  • Outer joins let you return all the data from one table and only some of the data in a second table that you join to it.
  • Full joins that let you display all the data from both tables in a join.
  • Intermediate joins are where you join multiple tables in a query, but not all the tables are used as data sources.
  • Self-joins are where you join a table to itself to display hierarchical data.
  • Joins on data ranges when you need to map a value in one table to a range of values in another table.
  • Joins on multiple fields.
  • Semi-joins show data from one table but filter using another table.
  • Cross-joins allow you to return all the data from both tables in a join.

  We realize that these concepts may seem more than a little abstract when you read about them for the first time. But do not worry; you will see how all these ideas have thoroughly practical applications to SQL queries and how they can be used in your day-to-day data analysis. Indeed, applying these more advanced techniques is often the only way to obtain clear and accurate results from SQL.

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  eBook - PDF

  Beginning SQL

  • Paul Wilton, John Colby(Authors)
  • 2005(Publication Date)
  • Wrox

    (Publisher)

  You can define a cross join in two ways. The first way of creating a cross join uses the CROSS JOIN statement, as shown in the following code, where the Category and Location tables are cross-joined: SELECT Category, Street FROM Category CROSS JOIN Location ORDER BY Street; The preceding syntax is supported by Oracle, MySQL, and SQL Server, but not by IBM’s DB2 or MS Access. An alternative syntax for a cross join is simply to list all the tables to be cross-joined in the FROM clause, as shown here: SELECT Category, Street FROM Category, Location ORDER BY Street; Virtually all databases support the second way of cross joining, so it’s likely the best way to perform a cross join. Both of the preceding queries do the same thing and produce the same results, which are shown here: Category Street Thriller Main Street Romance Main Street Horror Main Street War Main Street Sci-fi Main Street Historical Main Street Comedy Main Street Thriller Tiny Terrace Table continued on following page 213 Selecting Data from Different Tables Category Street Romance Tiny Terrace Horror Tiny Terrace War Tiny Terrace Sci-fi Tiny Terrace Historical Tiny Terrace Comedy Tiny Terrace Thriller Winding Road Romance Winding Road Horror Winding Road War Winding Road Sci-fi Winding Road Historical Winding Road Comedy Winding Road As you can see, the queries produce a lot of results given that the Location table has only three records and the Category table has only seven records. Why so many results? The results set is the Cartesian product of the two tables, which in plain English means all the rows from one table combined with all the rows from the other. This means that every single combination of the two tables appears in the results set. If you look at the results in the preceding table, you’ll see Main Street combined with Thriller, Main Street combined with Romance — indeed, Main Street combined with every record in the Category table.

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  eBook - ePub

  Beginning Microsoft SQL Server 2012 Programming

  • Paul Atkinson, Robert Vieira(Authors)
  • 2012(Publication Date)
  • Wrox

    (Publisher)

  UNION operator, which allows you to combine the results of two queries into one.

  COMBINING TABLE DATA WITH JOINS

  When you’re operating in a normalized environment, you’ll frequently run into situations in which not all of the information that you want is in one table. In other cases, all the information you want returned is in one table, but the information you want to place conditions on is in another table. This is where the JOIN clause comes in.

  A JOIN does just what it sounds like — it joins the information from two tables together into one result set. You can think of a result set as being a virtual table. It has both columns and rows, and the columns have data types. Indeed, in Chapter 7 , I’ll show you how to treat a result set as if it were a table and use it for other queries.

  How exactly does a JOIN put the information from two tables into a single result set? Well, that depends on how you tell it to put the data together — that’s why there are four kinds of JOIN s. The thing that all JOIN s have in common is that they match one record up with one or more other records to make a record that is a superset created by the combined columns of both records.

  For example, take a look at a record from a table called Films :

  FILMID	FILMNAME	YEARMADE
  1	My Fair Lady	1964

  Now follow that up with a record from a table called Actors :

  FILMID	FIRSTNAME	LASTNAME
  1	Rex	Harrison

  With a JOIN , you can create one record from these two records found in totally separate tables:

  This JOIN (at least apparently) joins records in a one-to-one relationship. One Films record joins to exactly one Actors record.

  Let’s expand things just a bit and see if you can see what’s happening. I’ve added another record to the Actors

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