Why using right outer join not logically read exactly? Overall, not a good idea, I strongly recommend against that. It would not read logically at all and be much harder to maintain and understand. I think a better techique is to always put inner joins followed by the outer joins.ĭaRage - to do that, you'd make a mess of this example and up needing a RIGHT OUTER JOIN at the end. That's why I wrote this post! To hopefully help explain it. > yes but you slap yourselft and correct the thing because you know WHY it's wrong. Unlike a ton of other "db developers" that have no clue about set theory happily writing up all those "complex" left joins. Yes but you slap yourselft and correct the thing because you know WHY it's wrong. So if I as an experienced developer occasionally trip up, I can see how it might initially be a concept that might not even occur to beginners. It's very easy to do and normally takes a few seconds before I slap my forehead and say Doh!. However I do sometimes find myself filtering out results with a clause on an outer join. > why on earth is this so hard for people to understand ? The big issue is just changing that second INNER JOIN to an OUTER JOIN which "seems" like it "fixes" things, but it really completely changes the logical definition of what you are returning. It seems like it might make sense at first, until you think about it. I can see how beginners might get confused, especially if they use the thought process I outlined in the article. Why on earth is this so hard for people to understand ? Re: Be Careful When Mixing INNER and OUTER Joins How to JOIN Multiple Transactional Tables in SQL.Better Alternatives to a FULL OUTER JOIN.If you use Derived Tables (or nested joins) the way you'd use parenthesis in a math or boolean equation to express your logical order of precedence, you can make your code cleaner and more readable and you can ensure that you get back the exact results you intended. So, be careful when mixing INNER and OUTER joins it can be tricky to hunt down unintended results and side effects. It's up to you to decide which option you'd prefer to use. In general, I have found that most database programmers tend to avoid nested joins expressions. That returns the correct results and is logically accurate, but I find it can be confusing to read and a little more difficult to maintain. Left outer join (Pets inner join PetTypes on Pets.PetTypeID = PetTypes.PetTypeID) This returns the results we are looking for: Then, we simply select FROM the People table and OUTER JOIN to the derived table. The best solution is to use a derived table to encapsulate the INNER JOIN between Pets and PetTypes. So, just changing INNER JOINS to OUTER JOINS to "fix" this problem may have unintended side effects, and your query is no longer logically the same! It may work in most cases, or in all cases depending on your constraints and relations, but it is important to understand that logically you have completely changed what you are asking for. Notice that suddenly Baby Puss appears! That is because we changed our join from Pets to PetTypes from an INNER to an OUTER. Left outer join PetTypes on Pets.PetTypeID = PetTypes.PetTypeID Left outer join Pets on Pets.OwnerID = People.PersonID The following statement uses the CROSS JOIN operator to join table T1 with table T2.Select People.PersonName, Pets.PetName, PetTypes.PetType ( 3) Code language: SQL (Structured Query Language) ( sql ) DROP TABLE IF EXISTS T1 ĬREATE TABLE T1 (label CHAR( 1) PRIMARY KEY) The following CREATE TABLE statements create T1 and T2 tables and insert some sample data for the cross-demonstration. INNER JOIN T2 ON true Code language: SQL (Structured Query Language) ( sql ) PostgreSQL CROSS JOIN example The following statement is equivalent to the above statement: SELECT select_listįROM T1, T2 Code language: SQL (Structured Query Language) ( sql )Īlso, you can use an INNER JOIN clause with a condition that always evaluates to true to simulate the cross-join: SELECT * The following illustrates the syntax of the CROSS JOIN syntax: SELECT select_listĬROSS JOIN T2 Code language: SQL (Structured Query Language) ( sql ) For example, the T1 has 1,000 rows and T2 has 1,000 rows, the result set will have 1,000 x 1,000 = 1,000,000 rows. If T1 has n rows and T2 has m rows, the result set will have nxm rows. Suppose you have to perform a CROSS JOIN of two tables T1 and T2. Introduction to the PostgreSQL CROSS JOIN clauseĪ CROSS JOIN clause allows you to produce a Cartesian Product of rows in two or more tables.ĭifferent from other join clauses such as LEFT JOIN or INNER JOIN, the CROSS JOIN clause does not have a join predicate. Summary: in this tutorial, you will learn how to use the PostgreSQL CROSS JOIN to produce a cartesian product of rows from the joined tables.
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