SQL Concepts-3

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  • In this blog we are going to understand some important queries...
  • Created new database and tables for solving following queries :


Union :

  • Union removes duplicate records from data sets.
  • Union first performs sorting operation and then eliminate duplicate records in data sets, after that they return combined data sets.
  • Example :

Union All :

  • Union All keeps all records from each data sets.
  • It also contains duplicate records.
  • Example :

Group By :

  • Group by is used to group the rows that have same values or it is used to organize similar data into groups.
  • Attribute i.e used in aggregate function cannot be in group by statement.
  • The attribute used after group by statement and attribute used in aggregate function are only allowed if rather than this another attribute used, it shows error :
  • Example :

Order By :

  • Order by is used to sort the data in ascending or descending order.
  • Example :

Subquery :

  • Subquery is the query within another query and included within WHERE clause.
  • The outer query is known as main query and inner query is known as subquery.
  • The subquery executes first then it returns result to outer query for condition checking.
  • It is easy to understand and slower than join.
  • Example :


Correlated subqueries :

  • It is used to fetch the outer query value for processing of inner query.
  • Again the inner query result is used for outer query processing.
  • This continues till matching result is found and then return to main.
  • It works like recursion.
  • It is used for calculating average.
  • Example :


Thank You!!!

For more understanding watch below video :
SQL

 








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