What are joins in oracle sql?

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What are joins in oracle sql?







It is said to be the query for combining the rows from two or more tables, views and materialized view. There must be at least one join condition in the from clause or where clause for combining the two tables. It will compare the two columns from the different tables and combines pair of rows included in the output which will satisfy the condition. Multiple tables can come after the FROM clause in the query. Select statement in the query can select any columns from the tables. 

If any two tables have the same column name, references as aliases form to represent the two different tables have the common column which are matching in terms of name and datatype. If the three tables i.e. table1, table2 and table3 are joined. First two tables are combined on the basis of common column parameters. In the same manner, the result of both tables i.e. table1 and table2 are combined with the third table on the basis of common column parameters.

It is the possible table expressions in a from clause which lay the foundation for the two tables combining.Explict equality test is used in the WHERE clause for this operation. [Where t1. col1=t2.col2 and t2. col2=t3.col3]. It is used to retrieve the data from the Mutiple tables. It combines the output from the two row sources and will produce the dataset i.e. single row source. Where clause is NON-ANSI standard and From clause is ANSI standard. If the statement is missing join keyword, database will perform automatically cartesian join on the statement in which each row of one table is multiplied with another row table.


What is the importance of SQL joins in database management?



It is very important in the field of database management. It allows us to combine the rows from two or more tables based on the related column, when working with normalized databases. They allow the data from the multiple tables to be queried together which is based on logical relationships between them. Mostly in the case of normalized databases, data is splitted to multiple related tables for reducing redundancy and improve data integrity.

Data is retrieving from multiple tables:

Today in these real-world applications, data are not present in the single table. It is very knowledge task to find out the data is present in which tables. Customer tables are joined with the order table to track the activity of list of customers who have purchased the particular order. 

Maintaining database normalization:

Improves flexibility:
It improves the complex queries flexibility by providing filters in the where clause and aggregate function. 

PLSQL blocks:

In PLSQL blocks, you can use the join in SELECT into statements or cursors.

Performance optimization:

It will improve the performance of the queries.

Business Intelligence:

EXPLAIN PLAN OR AUTOTRACE  is used to analyze the performance of join and indexes concept which are applied on the columns of the tables for optimizing the execution time.

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