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SQL Selectivity
Selectivity
Selectivity is used for the definition of the number of unique values in the table.
It shows how many unique values are there in the table.
Selectivity=Cardinality/number of record *100%
If the selectivity is high than onl...
Joins In SQL
Joins
There are different type of joins in SQL
1.Inner Join-Return all the values if there is a match in both the table.
2.Outer join
-left outer join-Return all the values from left table and the values matched from the right table.
...
SQL Joins
Joins
Joins in SQL are nothing but a technique to join the two table based on a common field.
OrderID CustomerID OrderDate
10308 2 1996-09-18
10309 37 1996-09-19
10310 77 1996-09-20
Custom...
Primary Key
Primary Key
A primary key in a table is a unique key that is the element in this column are unique and can not be null.
It is used to mantain the integrity.
It uniquely identify the row in the table.
It is a constraint on the table.
...
Indexing
The index is used to search the data in a table.
The index is basically a data structure that search our data.
It is applied usually on a column of a table.
It is usually a B-tree but can also be hash table and other depending upon the ...
Referential Integrity
Referential Integrity
Referential Integrity is a concept in which the tables shares a relationship between them and that should be consistent.
Ex:
If we have a table called Employee and the other Employee Salary and columns in them of na...
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint
This error generally comes when we want to delete child table row. To solve this problem we have to disable the
foreign key checks.
SET FOREIGN_KEY_CHECKS=0;
Now you can fire your query.
DELETE FROM TABLE WHERE ID = 'Somthing';
You can ena...
