Difference between a theta join equijoin and natural join

Question

I m having trouble understanding relational algebra when it comes to theta joins  equijoins and natural joins  Could someone please help me better understand it  If I use the   sign on a theta join is it exactly the same as just using a natural join

User · Answer

Natural is a subset of Equi which is a subset of Theta.

If I use the = sign on a theta join is it exactly the same as just using a natural join???

Not necessarily, but it would be an Equi. Natural means you are matching on all similarly named columns, Equi just means you are using '=' exclusively (and not 'less than', like, etc)

This is pure academia though, you could work with relational databases for years and never hear anyone use these terms.

User · Answer

Theta Join  When you make a query for join using any operator  e g       lt         etc    then that join query comes under Theta join   Equi Join  When you make a query for join using equality operator only  then that join query comes under Equi join   Example      SELECT   FROM Emp JOIN Dept ON Emp DeptID   Dept DeptID    SELECT   FROM Emp INNER JOIN Dept USING DeptID     This will show                                                       Emp Name   Emp DeptID   Dept Name   Dept DeptID                                                         Note  Equi join is also a theta join   Natural Join  a type of Equi Join which occurs implicitly by comparing all the same names columns in both tables     Note  here  the join result has only one column for each pair of same named columns   Example   SELECT   FROM Emp NATURAL JOIN Dept   This will show                                     DeptID   Emp Name   Dept Name

User · Answer

While the answers explaining the exact differences are fine  I want to show how the relational algebra is transformed to SQL and what the actual value of the 3 concepts is    The key concept in your question is the idea of a join  To understand a join you need to understand a Cartesian Product  the example is based on SQL where the equivalent is called a cross join as onedaywhen points out    This isn t very useful in practice  Consider this example   Product PName  Price                       Laptop    1500 Car       20000 Airplane  3000000   Component PName  CName  Cost                                Laptop  CPU     500 Laptop  hdd     300 Laptop  case    700 Car     wheels  1000   The Cartesian product Product x Component will be - bellow or sql fiddle  You can see there are 12 rows   3 x 4  Obviously  rows like  Laptop  with  wheels  have no meaning  this is why in practice the Cartesian product is rarely used        PNAME     PRICE    CNAME   COST   --------------------------------------     Laptop      1500      CPU    500       Laptop      1500      hdd    300       Laptop      1500     case    700       Laptop      1500   wheels   1000          Car     20000      CPU    500          Car     20000      hdd    300          Car     20000     case    700          Car     20000   wheels   1000     Airplane   3000000      CPU    500     Airplane   3000000      hdd    300     Airplane   3000000     case    700     Airplane   3000000   wheels   1000     JOINs are here to add more value to these products  What we really want is to  join  the product with its associated components  because each component belongs to a product  The way to do this is with a join       Product JOIN Component ON Pname   The associated SQL query would be like this  you can play with all the examples here   SELECT   FROM Product JOIN Component   ON Product Pname   Component Pname   and the result       PNAME   PRICE    CNAME   COST   ----------------------------------   Laptop    1500      CPU    500     Laptop    1500      hdd    300     Laptop    1500     case    700        Car   20000   wheels   1000     Notice that the result has only 4 rows  because the Laptop has 3 components  the Car has 1 and the Airplane none  This is much more useful    Getting back to your questions  all the joins you ask about are variations of the JOIN I just showed    Natural Join   the join  the ON clause  is made on all columns with the same name  it removes duplicate columns from the result  as opposed to all other joins  most DBMS  database systems created by various vendors such as Microsoft s SQL Server  Oracle s MySQL etc    don t even bother supporting this  it is just bad practice  or purposely chose not to implement it   Imagine that a developer comes and changes the name of the second column in Product from Price to Cost  Then all the natural joins would be done on PName AND on Cost  resulting in 0 rows since no numbers match    Theta Join   this is the general join everybody uses because it allows you to specify the condition  the ON clause in SQL   You can join on pretty much any condition you like  for example on Products that have the first 2 letters similar  or that have a different price  In practice  this is rarely the case - in 95  of the cases you will join on an equality condition  which leads us to    Equi Join   the most common one used in practice  The example above is an equi join  Databases are optimized for this type of joins  The oposite of an equi join is a non-equi join  i e  when you join on a condition other than      Databases are not optimized for this  Both of them are subsets of the general theta join  The natural join is also a theta join but the condition  the theta  is implicit    Source of information  university    certified SQL Server developer    recently completed the MOO  Introduction to databases  from Stanford so I dare say I have relational algebra fresh in mind

User · Answer

A theta join allows for arbitrary comparison relationships  such as  ge     An equijoin is a theta join using the equality operator   A natural join is an equijoin on attributes that have the same name in each relationship    Additionally  a natural join removes the duplicate columns involved in the equality comparison so only 1 of each compared column remains  in rough relational algebraic terms           pR S-as    aR aS

User · Answer

outis s answer is good  concise and correct as regards relations    However  the situation is slightly more complicated as regards SQL    Consider the usual suppliers and parts database but implemented in SQL   SELECT   FROM S NATURAL JOIN SP    would return a resultset   with columns  SNO  SNAME  STATUS  CITY  PNO  QTY  The join is performed on the column with the same name in both tables  SNO  Note that the resultset has six columns and only contains one column for SNO   Now consider a theta eqijoin  where the column names for the join must be explicitly specified  plus range variables S and SP are required    SELECT   FROM S JOIN SP ON S SNO   SP SNO    The resultset will have seven columns  including two columns for SNO  The names of the resultset are what the SQL Standard refers to as  implementation dependent  but could look like this   SNO  SNAME  STATUS  CITY  SNO  PNO  QTY  or perhaps this   S SNO  SNAME  STATUS  CITY  SP SNO  PNO  QTY  In other words  NATURAL JOIN in SQL can be considered to remove columns with duplicated names from the resultset  but alas will not remove duplicate rows - you must remember to change SELECT to SELECT DISTINCT yourself         I don t quite know what the result of SELECT   FROM table expression  is  I know it is not a relation because  among other reasons  it can have columns with duplicate names or a column with no name  I know it is not a set because  among other reasons  the column order is significant  It s not even a SQL table  or SQL table expression  I call it a resultset

User · Answer

Cartesian product of two tables gives all the possible combinations of tuples like the example in mathematics the cross product of two sets   since many a times there are some junk values which occupy unnecessary space in the memory too so here joins comes to rescue which give the combination of only those attribute values which are required and are meaningful    inner join gives the repeated field in the table twice whereas natural join here solves the problem by just filtering the repeated columns and displaying it only once else  both works the same  natural join is more efficient since it preserves the memory  Also   redundancies are removed in natural join    equi join of two tables are such that they display only those tuples which matches the value in other table   for example    let new1 and new2 be two tables   if sql query select   from new1 join new2 on new1 id   new id  id is the same column in two tables  then start from new2 table and join which matches the id in second table   besides   non equi join do not have equality operator they have  lt    and between operator    theta join consists of all the comparison operator including equality and others  lt       comparison operator  when it uses equality    operator it is known as equi join

User · Answer

Natural Join  Natural join can be possible when there is at least one common attribute in two relations   Theta Join  Theta join can be possible when two act on particular condition   Equi Join  Equi can be possible when two act on equity condition  It is one type of theta join

[sql] Difference between a theta join, equijoin and natural join

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