# What is semi-join technique of distributed system?

## What is semi-join technique of distributed system?

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

### What are semi-join based algorithms?

Semi-join is a relational operator that decreases the cost of processing queries involving binary operations. This is accomplished by initially selecting the data relevant to answer the queries and thereby reducing the size of the operand relations.

What is the main purpose of semi-join in distributed processing system?

Query optimization and processing is one of the key technologies in distributed database system. It generally uses semi-join operation to improve the time response performance of query and reduce communication cost.

What is semi-join in advanced DBMS?

Semi-join is introduced in Oracle 8.0. It provides an efficient method of performing a WHERE EXISTS sub-query. A semi-join returns one copy of each row in first table for which at least one match is found. Semi-joins are written using the EXISTS construct.

## What is semi join give a suitable example?

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). No row duplication can occur. Regular join duplicates rows if there are multiple matches on the join predicate. Semi join is defined to only return columns from input A.

### What is left semi join?

A LEFT SEMIJOIN (or just SEMIJOIN ) gives only those rows in the left rowset that have a matching row in the right rowset. The RIGHT SEMIJOIN gives only those rows in the right rowset that have a matching row in the left rowset. The join expression in the ON clause specifies how to determine the match.

What is difference between join and Semijoin?

In context|algebra|lang=en terms the difference between semijoin and join. is that semijoin is (algebra) a type of join in relational algebra, denoted by while join is (algebra) the lowest upper bound, an operation between pairs of elements in a lattice, denoted by the symbol.

What is the difference between inner join and semi join?

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. Then a LEFT SEMI JOIN is the appropriate query to use.

## How does left semi join work?

### What is the difference between semi join and inner join?

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.

How to use semijoin in distributed query processing?

References 1. Using 2-way semijoin in distributed query processing. By Hyunchul Kang and Nick Roussopoulos. 2. Improving distributed query processing by hash- semijoins. By Judy Tseng and Arbee Chen. 3. Domain Specific Semijoin:A new operation for distributed query processing. By Jason Chen and Victor Li.

What is the purpose of semi join in SQL?

Semi-Join •A Join where the result only contains the columns from one of the joined tables. •Useful in distributed databases, so we don’t have to send as much data over the network. •Can dramatically speed up certain classes of queries.

## What does semi join mean in relational algebra?

Join  Join is a binary operation in Relational Algebra.  It combines records from two or more tables in a database.  A join is a means for combining fields from two tables by using values common to each. 6. Semi-Join •A Join where the result only contains the columns from one of the joined tables.

### How to use semi join in SlideShare query?

Query written with a conventional join: SELECT D.deptno, D.dname FROM dept D, emp E WHERE E.deptno = D.deptno ORDER BY D.deptno; ◦ A department with N employees will appear in the list N times. ◦ We could use a DISTINCT keyword to get each department to appear only once. 9.