What is a semi join?

Asked By: Devabhuti Burchhardt | Last Updated: 1st May, 2020
Category: technology and computing databases
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A semi join returns a row from one join input (A) if there is at least one matching row on the other join input (B). Regular join duplicates rows if there are multiple matches on the join predicate. Semi join is defined to only return columns from input A.

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Similarly one may ask, what is the purpose of a semi join?

Semijoin is a technique for processing a join between two tables that are stored sites. The basic idea is to reduce the transfer cost by first sending only the projected join column(s) to the other site, where it is joined with the second relation.

Also Know, what is hive semi join? name)) If there are multiple matching rows in the right-hand column, an INNER JOIN will return one row for each match on the right table, while a LEFT SEMI JOIN only returns the rows from the left table, regardless of the number of matching rows on the right side.

Considering this, what is anti semi join?

An anti-join is essentially the opposite of a semi-join: While a semi-join returns one copy of each row in the first table for which at least one match is found, an anti-join returns one copy of each row in the first table for which no match is found.

What is Advanced Semi join database?

Semi-join strategies are technique for query processing in distributed database systems. ?A semi-join between two tables returns rows from the first table where one or more matches are found in the second table.

25 Related Question Answers Found

What are the different types of joins?

There are four basic types of SQL joins: inner, left, right, and full. The easiest and most intuitive way to explain the difference between these four types is by using a Venn diagram, which shows all possible logical relations between data sets.

What is natural join?

A NATURAL JOIN is a JOIN operation that creates an implicit join clause for you based on the common columns in the two tables being joined. Common columns are columns that have the same name in both tables. A NATURAL JOIN can be an INNER join, a LEFT OUTER join, or a RIGHT OUTER join. The default is INNER join.

What is an inner join SQL?

What is Inner Join in SQL? The INNER JOIN selects all rows from both participating tables as long as there is a match between the columns. An SQL INNER JOIN is same as JOIN clause, combining rows from two or more tables.

What is Theta join in DBMS?

A theta join is a join that links tables based on a relationship other than equality between two columns. A theta join could use any operator other than the “equal” operator.

How does hash join work?


In a HASH join, Oracle accesses one table (usually the smaller of the joined results) and builds a hash table on the join key in memory. It then scans the other table in the join (usually the larger one) and probes the hash table for matches to it.

What is relational algebra in DBMS?

What is Relational Algebra? Relational algebra is a widely used procedural query language. It collects instances of relations as input and gives occurrences of relations as output. It uses various operation to perform this action. Relational algebra operations are performed recursively on a relation.

How use outer join in SQL?

SQL left outer join is also known as SQL left join. Suppose, we want to join two tables: A and B. SQL left outer join returns all rows in the left table (A) and all the matching rows found in the right table (B). It means the result of the SQL left join always contains the rows in the left table.

What is correlated subquery in SQL?

In a SQL database query, a correlated subquery (also known as a synchronized subquery) is a subquery (a query nested inside another query) that uses values from the outer query. Again, because the subquery is correlated with a column of the outer query, it must be re-executed for each row of the result.

What is left anti join?

A left anti join returns that all rows from the first table which do not have a match in the second table.

What left anti?


Instead of returning rows when there is a match (according to the join predicate) between the left and right side, an anti-join returns those rows from the left side of the predicate for which there is no match on the right.

What is cross join?

A cross join is used when you wish to create combination of every row from two tables. All row combinations are included in the result; this is commonly called cross product join. A common use for a cross join is to create obtain all combinations of items, such as colors and sizes.

What is left outer join in hive?

Left Outer Join:
Hive query language LEFT OUTER JOIN returns all the rows from the left table even though there are no matches in right table. If ON Clause matches zero records in the right table, the joins still return a record in the result with NULL in each column from the right table.

What is the default join in hive?

1 Answer. Hive supports equi joins by default. You can optimize your join by using Map-side Join or a Merge Join depending upon the size and sort order of your tables.

What is MAP side join?

Map side join is a process where joins between two tables are performed in the Map phase without the involvement of Reduce phase. Map-side Joins allows a table to get loaded into memory ensuring a very fast join operation, performed entirely within a mapper and that too without having to use both map and reduce phases.

Which is faster map side join vs reduce side join Why?


Map side join is usually used when one data set is large and the other data set is small. Whereas the Reduce side join can join both the large data sets. The Map side join is faster as it does not have to wait for all mappers to complete as in case of reducer. Hence reduce side join is slower.

What are the types of joins in hive?

Moreover, there are several types of Hive join – HiveQL Select Joins: Hive inner join, hive left outer join, hive right outer join, and hive full outer join. We will also learn Hive Join tables in depth.

How do you join in hive?

How to Perform Joins in Apache Hive
  1. INNER JOIN – Select records that have matching values in both tables.
  2. LEFT JOIN (LEFT OUTER JOIN) – Returns all the values from the left table, plus the matched values from the right table, or NULL in case of no matching join predicate.