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Showing entries 1 to 7

Displaying posts with tag: inner join (reset)

MySQL DELETE Join example
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A very useful helper in your join toolbox can be a delete join. Even though it’s not a special join type but a join used within DELETE statements, it’s still worth mentioning. However, from time to time when I want to make use of delete joins  on my own, I somehow managed it to forgot the syntax and have to look it up somewhere. Therefor, here is a description as well as an example.

Take care: A delete join is a very powerful weapon. And with great power comes great responsibility! I recommend to develop the delete join on a development database. At least, make sure you have a working an recent backup before trying to delete things. A delete statement that uses joins makes it easy to shot yourself in the foot. And if you do, it probably blows away your hole leg.

 

DELETE  [Read more...]

Joins in MySQL 5: #1054 – Unknown column ‘…’ in ‘on clause’
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If you used to write MySQL joins for MySQL versions < 5.0 or upgrade your server from MySQL 4 to MySQL >= 5.0 you maybe run into a problem when you execute the following query:

SELECT *
FROM mytable1, mytable2
INNER JOIN mytable3
ON mytable1.mycolumnname = mytable3.mycolumnname
WHERE mytable1.id = mytable2.id;

#1054 – Unknown column ‘mytable.mycolumnname’ in ‘on clause’

Even though you made sure that the column exists, the problem persists. It can be a very annoying and time-consuming task to track this kind of error down to it’s cause: MySQL starting from version 5.0 tries to be more compliant to ANSI SQL. The tables are beeing joined in a different order. The solution to this problem is actually very simple. Surround the tables in

  [Read more...]
MySQL INNER JOIN Tutorial: the comma operator
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Here you find information about writing inner joins with the comma operator. It’s the most basic way to combine (join) two tables. There is an alternative syntax that can be used, because in MySQL you can write inner joins in two different ways. Another popular way is it to use the INNER JOIN command or synonymous keywords like CROSS JOIN and JOIN. Please make sure to read our dedicated documentation for more information when you understand the comma operator syntax.

Syntax

The following examples are equivalent to the INNER JOIN examples, to make it easy to

  [Read more...]
MySQL NATURAL JOIN Tutorial & Examples
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This tutorial explains how you can use NATURAL JOINs and what a natural join is actually. Included are syntax details and example statements.

Generally speaking, the keyword NATURAL can be treated as a join condition which is added implicitly. If used, it replaces the keywords ON and USING altogether. In MySQL writing natural joins means adding the keyword NATURAL to either an INNER JOIN or an OUTER JOIN. Let’s take a look at how a natural join implies a join condition.

Syntax

First of all, some natural join syntax examples. As mentioned earlier, a natural join adds a implicit condition to inner and outer join statements:

-- natural inner join
SELECT

  [Read more...]
MySQL STRAIGHT JOIN Tutorial & Examples
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This tutorial shows you how to write straight joins in MySQL and when an it makes sense to do so. Included are a general description, syntax examples and a comparison of straight and inner joins.

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Straight Join

A STRAIGHT_JOIN identifies and combines matching rows which are stored in two related tables. This is what an inner join also does. The difference between an inner join and a straight join is that a straight join forces MySQL to read the left table first.

To see if the STRAIGHT_JOIN hint would make sense, you have to use EXPLAIN to analyze your queries. When you feel that another join order could improve the performance, use a straight join.

Left table?

The STRAIGHT_JOIN keyword or hint, forces

  [Read more...]
MySQL INNER JOIN Tutorial & Examples
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This tutorial shows you how to write ANSI-Style inner joins with the INNER JOIN keywords. Included are a general description, some syntax examples and a comparison between inner and cross joins.

Note: In MySQL the join keywords JOIN and CROSS JOIN are synonymous with INNER JOIN. That means: All example statements found in this article work fine when you replace INNER JOIN with JOIN or CROSS JOIN.


Syntax

Here are syntax examples for the impatient. Basically, ANSI-style join conditions can be specified with two different keywords: USING and ON. Take a look at the following examples:

-- inner join with USING clause
SELECT *
FROM <firstTable> a INNER JOIN <anotherTable> b
USING(<columnName>)


  [Read more...]
Good SQL Querying
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By “Good SQL Querying”, I am not referring to “how to make your queries more perfomant.” I am about to go on a mini-rant about how to make readable and self-documenting SQL queries.

One practice that will get me instantly going on a rant is using a comma join. There is NO reason to do the following:

-- uses the sakila sample database
SELECT first_name, last_name, address
FROM customer,address;

What kind of join did the original author intend? A CROSS JOIN? Or did they really want an INNER JOIN and forget the WHERE clause?

The answer: you do not know for sure; you can only guess. Had the query been

SELECT first_name,last_name,address
FROM customer INNER JOIN address;

you would know that the author intended an INNER JOIN; had the query been

SELECT first_name,last_name,address
FROM customer CROSS JOIN
  [Read more...]
Showing entries 1 to 7

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