SQL - Wikitechy

Can you use UNIQUE KEY as a replacement for Primary Key ?

webmaster — Thu, 29 Sep 2022 09:17:56 +0000

Both Keys are different. For Identifying each and every record in a table we use primary key.
To apply additional uniqueness for other columns on the table we use Unique Key.
Primary Keys are important for a table.
Internal users use Primary Key whereas external users use unique key as it contains some meaningful information.
When uniqueness is needed for multiple columns for a table, we use unique key for all columns and primary keys cannot be used in this case.
Primary key uses clustered index by default and unique key uses Non-clustered index.

What are all the different attributes of indexes ?

webmaster — Thu, 29 Sep 2022 09:06:19 +0000

The indexing has various attributes:

Access Types:

Value based search , range acess can be refered using Access types.

Access Time:

Time that is needed to find a data element or a set of elements are called Access Time.

Insertion Time:

Time needed to find an approriate space and insert a new data is known as Insertion Time.

Deletion Time:

Time needed to find an element and delete that element as well as update its index structure is known as Deletion Time.

Space Overhead:

Index always needs additional space which is provided by the space overhead.

What are the different types of SQL commands ?

webmaster — Tue, 27 Sep 2022 10:48:22 +0000

DDL (Data Definition Language)
DML (Data Manipulation Language)
DCL (Data Control Language)
TCL (Transaction Control Language)

DDL(Data Definition Language)

DDL is a syntax that is used for creating SQL commands. It is called as DDL becuase it defines the syntax for creating a data base.
DDL is part of SQL which defines the data structure of the database at the initial stage when creating a database and is also used for creating and modification of the structure of the database.

Commands in DDL are

Create table: This command is used to create a table in a database or its objects (like table, index, function, views, store procedure, and triggers).
Alter table: This is used to alter the structure of the database.
Drop table: This command is used to delete objects from the database.

Data Manipulation Language

DML uses the existing data base for manipulation of data.
It helps users to retreive the data and perform manipulation on the data like inserting data, updating data, and deleting data using the following data manipulation commands like insert, update and delete.

Data Control Language

As the name suggests data control language provides control for accessing the data in the database.
These commands are usually used for providing previlages and controls to users who likes to access the database.
Commands used in DCL for control and user previlages are Grant and Revoke.

Transaction Control Language

TCL command statements are used to control changes made by the DML commands.
It also authorizes the statements to assemble in conjunction into logical transactions.
The commands that are used in TCL are Commit, Rollback, Savepoint, Begin, and Transaction.

Difference between SQL and PLSQL

webmaster — Tue, 27 Sep 2022 10:40:47 +0000

SQL	PL/SQL
SQL is structural query language for database. It has no variables.	Pl/SQL is a programming language using SQL for database. It has variables and data types
SQL has no if condition, loops and similar structures	Pl/SQL has for loops, if condition, while loop and other similar structures
SQL can execute single command at a time	It can perform multiple operations at a time
SQL is a declarative language	It is a procedural language
SQL can be embedded in PL/SQL block	Can also be embedded in SQL code
SQL directly interacts with database server.	Pl/SQL does not directly interact with database server.
SQL is a data oriented language	Pl/SQL is an application oriented language
SQL is used to write queries, create and execute DDL and DML Commands	Pl/SQL is used to write program blocks, functions, procedures,triggers and packages

Char vs Varchar in SQL

webmaster — Tue, 27 Sep 2022 09:31:25 +0000

Char	VarChar
Char data type is used for storing single character values	It is a SQL Code that helps in storing variable character
Used to store characters string of fixed length specified	If a length of a string is less than the set or fixed length then it will store as it is without including any extra blank spaces.
Size is fixed. Cannot be changed during runtime	Storage size of VARCHAR datatype is equal to the actual length of the entered string in bytes.
Can Store only characters	We should use this datatype when we expect the data values in a column are of variable length.
Memory is allocation is Static	Memory is allocated Dynamically
Per Character contains 1 byte	For Storing it uses 1 byte per character Plus 1 or 2 extra bytes for stroing varying length.
Since the size of the char is fixed and cannot be changed, there will be a memory wastage.	To store a character string value of set length then varchar data type is used

What is Default constraint in SQL ?

webmaster — Tue, 27 Sep 2022 06:00:52 +0000

Constraints are a certain set of rules used for processing data and limiting the type of data that can go into the table.
The default constraint is used to define a default value for a column so that the default value will be added to all new records if no other value is specified.
For example, if we set the default value to 85000 as a default constraint for the E_salary column, then all the other entries in the column will have a default constraint value of 85000 until no other values have been assigned for default constraint during insertion.

Syntax for setting a default constraint. Start by creating a new table and adding a default constraint to one of its columns.

create table stu1(s_id int, s_name varchar(20), s_marks int default 50)

What are the different types of relationships in SQL ?

webmaster — Tue, 27 Sep 2022 05:48:21 +0000

Data base collects different types of data that uses tables, record, and columns.
For easy storage of data, a well-defined relationship is created between tables and databases.
Data in a table is based on a relational model.
A unique Id and the key is created for each row.
Data attributes are held in the column of the table.

Different Types of Relational DataBase Tables

One to One Relationship

This relationship helps to create a relationship between tables where a single row of the first table can only be related to one and only one record of the second row.
The same way one record of the second table can be related to one and only one record of the first table.

One to many Relationship

This helps to create a relationship between the two tables.
Any single row of a table can be connected to one or more rows in a second table.
Rows from the second table can relate to only one row in the first table. This relationship is known as one to many relationship.

Many to Many Relationship

Here relationship is created between two tables.
Each record in the first table can be related to many records in the second table.
Similarly, each record from the second row can relate to any records in the first row.
It is represented as N: N.

Types of indexes in SQL

webmaster — Mon, 26 Sep 2022 11:36:47 +0000

There are various types of indexes in SQL server:

Clustered Index
Non-Clustered Index
Column Store Index
Filtered Index
Hash Index
Unique Index

Clustered Index

Rows of data are sorted and stored data in a table or view depending on the central values by clustered index.
Each table can have one clustered index as it enables the user to store the data in a single order.
Data is sorted and stored in gathered index, so if we see a data in a table sorted means it is arranged with clustered index.
If a table contains clustered index, it is called as clustered table.
When a huge data needs to mdified in a database we use clustered index.

Non Clustered index

Non clustered index are structures that are seperated from the data row.
This type of index contains non clustered key values and those pairs has a pointer to that data row.
In non clustered index, arrow from an index row to data row is called as row locator.
Row locator structure identifies whether the data pages are in the form of clustered table or heap.
Row locator is termed as cluster index key in clustered chart.
User can easily add non key columns to the leaf level in a non clustered index as it byepasses the existing index key limits to perform fully covered indexed queries.
A non clustered index is created to improve the overall performance of frequently asked questions which cannot be created in clustered items.

Unique Index

It enables users to confirm that every row in the table is unique in one way or other by checking if the indexing key contains any duplicate values.
When user wants to use the unique character of data he can use unique index.
Unique index allows indidviduals to ensure that data integrity of each defined column of the table in the database.
It provides additional information about the data table which is helpful for query optimizer.

Filtered Index

This index is created when column has only a small number of relevent values for queries on the subset of values.
When table contains hetrogeneous data rows, a filtered index is created in sql for one or more data types.
Filtered index is indicate even when query optimizer does not cover any query.
Filtered index indexes the portion of rows in a table, it applies filter on index by improving the overall performance of the query.
Filtered index helps to reduce the maintanence cost and storage cost
Overall impact of data modification is less in filtered index as it is updated only when a new record is inserted or when the data of the index is impacted.

Column Store Index

Column store index is standard form of index when it comes to storing and querying the large data warehousing tables.
This index was designed for the impovement in the performance of query in case of workloads with vast data.
Data are stored as column based format.
It reduces overall storage cost and provides high level compression of data.
Since data compression happens, user can efficiently perform input and output functions for a high performance result.

Hash Index

Hash index is simply an array in sql that contains pointer slot and a row on each slot.
Hash index use hash function F(K,N) where K is critical and N is number of Buckets.
Function maps out to key corresponding to the bucket of the hash index.
Each hash bucket index contains 8 bytes which is used to store the memory address of the linked list of critical entities.

What is the difference between BETWEEN and IN operators in SQL ?

webmaster — Mon, 26 Sep 2022 09:59:45 +0000

BETWEEN

This condition allows you to check if given values are in the specified range.

In operator is used for matching values in the given list of values.

Values can be text, date or numbers.

In operator can be used to remove multiple or condition in select, Insert, update, or delete.

Between commands can be used in Select, Insert, update or delete.

Not In is used to exclude rows in the list.

This condition will return records between the range of values 1 and 2.

Duplicate entries are retained.

Syntax:

SELECT Fname, Lname

FROM Employee

WHERE Salary

BETWEEN 30000 AND 45000;

Syntax:

SELECT Fname, Lname

FROM Employee

WHERE Salary IN (30000, 40000, 25000);

What is a join in SQL ? What are the types of joins ?

webmaster — Mon, 26 Sep 2022 09:24:54 +0000

Data is stored in multiple tables that are related to each other in relational data bases like SQL Server, MySQL etc with a common key value.
When we need to get row by combining one or more tables based on common fields between them can be achieved using a SQL Join.
When we need to extract records from two or more tables into a new table, JOIN clause helps us to acheive this based on certain condition.
Based on logical relationship between tables join clause helps us to query and access data from multiple tables.
In other words, Join show how data from a row can be fetched from one table to another table.

Different types of Joins are:

Inner join/Simple Join
Left Outer join/Left Join
Right Outer Join/Right join
Full Outer join
Cross Join
Self Join

Inner Join

In SQL if a defined condition is valid the inner join will select all the matching rows and columns from both tables.

Syntax

Select column_1, column_2 column_3 from table_1 innerjoin table_2 ON table_1.column = table_2.column;

Left Join

In left join all records are retrived from left table(table1) and matched rows and column from right table(table2).
If there is no matching rows or columns, left join returns NULL.

Syntax

Select column_1, column_2, columns from Table_1 left join table_2 ON table_1.column_name = table_2.column name.

Right Join or Right Outer Join

Records from the right table are retrived in RIGHT JOIN and the matched rows or columns from the left table(table1).
If there is no matching rows or columns it will return NULL.

Syntax

Select column_1, column_2, cloumn(s) FROM table_1 right join table_2 ON table_1.column_name= table2.column_name.

Full Join or Full Outer Join

This join is a combination of both Left join and right join.
The joined tables returns all the records from both the tables.
If no matches are found in the table, this join will return a NULL Value.
It is also called as Full Outer Join.

Syntax

Select column_1, column_2, column(s) from table_1 Full join table_2 ON table_1.column_name = table_2.column_name;

Cross Join

It is also known as Cartesian Join which returns cartesian product of two or more joined tables.
Cross join produces a table that merges each rows from the first table with each rows of second table.
No condition ir required for CROSS JOIN.

Syntax

Select * from table_1 cross join table_2;  
Or,
Select column1, column2, column3 from table_1, table_2;

Self Join

Tables formed by joining itself is called Self Join.
It makes a temporary naming of atleast one table in SQL Statement.

Syntax

Select column1, column2, column(s) from table_1 Tbl1, table_2 Tbl2 where condition;

What are the different types of database management systems ?

webmaster — Sat, 20 Aug 2022 08:25:40 +0000

Database Management System is classified into four types:

Hierarchical Database

It is a tree like structure where the data is stored in form of a tree.
In this type of database the parent may have many children but child should have single parent.

Network Database

This data base is represented as graph that allows many to many relationships.
In this data base children can have multiple children.

Relational Database

This relational database is represented in the form of a table.
Values in rows and column are associated with each other. This database is widely used as it is very easy to use.

Object-Oriented Database

In this data base everything is stored as objects.
All the objects in this database will have multiple relationships among them.

What is a Database ?

webmaster — Sat, 20 Aug 2022 08:14:29 +0000

In computing, database is an organized collection of data stored which is accessed electronically.
A database is usually controlled by Data Base Management Systems(DBMS).
In a computer data is stored in a structured form which is known as a Database.
Database is a collection of data in a organized manner and it can be accessed in several ways.Database contains collection of schemas, tables, queries and views.
Databases help us in storing, accessing and manipulating the data stored in a computer.
The Database Management System allows users to directly communicate with the database.
Small databases can be stored on file system but larger databases need computer clusters or cloud storage.
Database design techniques includes modelling of data, data is stored efficiently, sensitive data is secure and private.
DBMS-Data Base Managment Services is a software that communicates with end users, applications and the database itself to analyse and maintain the data.

What is a primary key ?

webmaster — Sat, 20 Aug 2022 07:55:57 +0000

In relational database primary key is a specific choice of minimal set of attributes that uniquely specify a relation.
Primary key is a special realational database table column key that uniquely identifies a table record.
In a table if primary key needs to be created then we have to define a primary key constraint when you create or modify a table.
When multiple coulmns are declared as primary key then it is called as Composite key.
A table should not contain more than one primary key.

A primary key’s main features are:

It must contain a unique value for each row of data. A table can have only one primary key constraint
It cannot contain null values.
Every row must have a primary key value.
It enforces the entity integrity of data.
Primary key always has a unique data.
Primary key length cannot be more than 900 bytes.
Primary key cannot have a null value.
Duplicate values are not allowed in primary key.
A primary key might use one or more fields already present in the table, or a unique field can be created to be the primary key.

What is Normalization in SQL ?

webmaster — Sat, 20 Aug 2022 07:27:08 +0000

It is a rule where larger tables are divided into smaller tables and they are linked using relationships.
Process of organizing data in a data base is called as Normalization.
The purpose of Normalisation is to eliminate repetitive data and ensure that data is stored logically.
Normalization minimizes redundency from a relation or relation set.
Normalization also helps in minimizing Reuse of data.
Also helps in reduction of insertion, updation and deletion anamolies.
Relation schemas that do not meet the properties are decomposed into small relation schemas so that they can meet the desired properties.
It is a rule where larger tables are divided into smaller tables and they are linked using relationships.
Database table redundancy is reduced by using normal forms.

What is SQL ?

webmaster — Sat, 20 Aug 2022 07:00:04 +0000

SQL stands for Structured Query Language. It is used for interacting with databases.
SQL is also known as standard query language which is used for maintaining relational database management systems(RDBMS).
SQL is a subject specific language which is used for programming and designed for maintaining data.
It performs different operations of data manipulation on different types of data using features of SQL.
It is used for creation and deletion of database.
It can also be used to fetch and modify rows and tables in a database.
SQL is specialized language for updating, deleting,and requesting information from databases.

Lazy writer in Memory ?

Editor — Sun, 18 Jul 2021 20:31:13 +0000

Lazy write

Lazy write is an effect caused by the use of a write-back cache. In a write-back cache, data is first written to the cache and then is later (during a particular time such as an idle machine cycle) is written to the disk or memory.
This type of cache is more complex to implement, due to the need to track each locations that have been written over and what needs to be marked as dirty to fix later.

Lazy writer in Memory

Lazy writer is a thread which is present for each NUMA node (and every instance has at least one) that scans through the buffer cache associated with that node.
The lazy writer thread sleeps for a specific interval of time, and when it wakes up, it examines the size of the free buffer list.
If the list is below a certain threshold, which depends on the total size of the buffer pool, the lazy writer thread scans the buffer pool to repopulate the free list.
As buffers are added to the free list, they are also written to disk if they are dirty.
So if there is no memory pressure and memory notification is stable it sleeps
The sole purpose of Lazy Writer is to maintain some free buffers in the SQL Server Buffer Pool.
Lazy writer runs periodically and check which buffers can be flushed and returned to the free pool.
So even SQL Server is not under memory pressure, it will also work regularly.

What is the difference between the SubQuery and Corelated SubQuery ?

Editor — Sun, 18 Jul 2021 20:30:02 +0000

Subquery

The inner query is executed only once.
The inner query will get executed first and the output of the inner query used by the outer query.
The inner query is not dependent on outer query.

SELECT Employee_name, dept_no 
FROM Employee 
WHERE Employee_name IN (SELECT Employee_name FROM Customer);

Correlated SubQuery

The outer query will get executed first and for every row of outer query, inner query will get executed.
So the inner query will get executed as many times as number of rows in result of the outer query.
The outer query output can use the inner query output for comparison. This means inner query and outer query dependent on each other.

SELECT Employee_name,dept_id 
FROM Employee
WHERE Employee_name IN (SELECT Employee_name FROM dept WHERE Employee.dept_id=dept.dept_id);

What is the Difference between join and union ?

Editor — Sun, 18 Jul 2021 20:27:46 +0000

Union

- The union operator combines the results of two or more queries into a single result set. But no.of columns must match in both/all the queries (and also the order) which are used for union.
- We can union two tables irrespective of common field.
- The UNION set operator is used for combining data from two tables which have columns with the same datatype.
  - Union – Returns with no duplicate rows.
  - Union all – Retruns with duplicate rows (No. of rows returned = No. of rows in Query1 + No. of rows in Query 2).

We cannot use the union operator within a create view statement.
We cannot use the union operator on text and image columns.

Syntax

SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions]
UNION
SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions];

Join

Joins are used to extract information from more than one table based on the related column/coloums (PK and RFK) any no. of rows can be retrived based on matching colums.
We can join two tables by ‘join’ if they have common field.
A Join is used for displaying columns with the same or different names from different tables.
Different types of joins are Inner join=equi join=join , outer join (Right outer join/right join, Left outer join / left join), Cross join , and full outer join/outer join.
We can use join operator within a create view statement.
We can use the join operator on text and image columns.

Syntax

SELECT column-names
FROM table-name1 JOIN table-name2 
ON column-name1 = column-name2
WHERE condition

What is the SQLCode -501 ?

Editor — Sun, 18 Jul 2021 20:24:32 +0000

What is the SQLcode -501 ?

SQLcode -501 pops up to show that , the Cursor identified in fetch or close statement is not opened.

SQL error code means it return codes that are preceded by a minus sign (-) indicate that the SQL statement execution was unsuccessful.
- If SQLCODE = 0, execution was successful.
- If SQLCODE > 0, execution was successful with a warning.
- If SQLCODE < 0, execution was not successful.
- SQLCODE = 100, “no data” was found.

What is the difference between a HAVING CLAUSE and a WHERE CLAUSE ?

Editor — Sun, 18 Jul 2021 20:21:49 +0000

Where Clause	Having Clause
WHERE clause can be used with – Select, Insert, and Update statements.	HAVING clause can only be used with the Select statement.
The WHERE search condition is applied before the grouping operation occurs.	The HAVING search condition is applied after the grouping operation occurs.
Aggregate functions cannot be used in the WHERE clause, unless it is in a sub query contained in a HAVING clause.	Aggregate functions can be used in Having clause.

Example of WHERE and HAVING clause

We have two tables Wikitechy_Employee and Department. Wikitechy_Employee contains details of employees e.g. id, name, age, salary and department id, while Department contains id and department name.
In order to show, which employee works for which department we need to join two tables on DEPT_ID to get the the department name.
Our requirement is to find how many employees are working in each department and average salary of department.
In order to use WHERE clause, we will only include employees who are earning more than 5000.
Before executing our query which contains WHERE, HAVING, and GROUP BY clause, let see data from Wikitechy_Employee and Department table:

SELECT * FROM Wikitechy_Employee;

EMP_ID	EMP_NAME	EMP_AGE	EMP_SALARY	DEPT_ID
1	George Orwell	23	10000	1
2	J.K. Rowling	24	7000	2
3	Kurt Vonnegut	25	8000	3
4	Virginia Woolf	27	6000	1
5	Ernest Hemingway	28	5000	2

SELECT * FROM Department;

DEPT_ID	DEPT_NAME
1	Accounting
2	Marketing
3	Sales

Sample Code

 
SELECT d.DEPT_NAME,
count(e.EMP_NAME) as NUM_EMPLOYEE, 
avg(e.EMP_SALARY) as AVG_SALARY FROM Employee e,
Department d WHERE e.DEPT_ID=d.DEPT_ID AND EMP_SALARY > 5000 GROUP BY d.DEPT_NAME;

Output

DEPT_NAME	NUM_EMPLOYEE AVG_SALARY	AVG_SALARY
Accounting	1	8000
Marketing	1	7000
Sales	2	8000

From the number of employee (NUM_EMPLOYEE) column you can see that only Vijay who work for Marketing department is not included in result set because his earning 5000.
This example shows that, condition in WHERE clause is used to filter rows before you aggregate them and then HAVING clause comes in picture for final filtering, which is clear from following query, now Marketing department is excluded because it doesn’t pass condition in HAVING clause i..e AVG_SALARY > 7000

Sample Code

 
SELECT d.DEPT_NAME,
count(e.EMP_NAME) as NUM_EMPLOYEE,
avg(e.EMP_SALARY) as AVG_SALARY FROM Employee e,
Department d WHERE e.DEPT_ID=d.DEPT_ID AND EMP_SALARY > 5000 GROUP BY d.DEPT_NAMEHAVING AVG_SALARY > 7000;

Output

DEPT_NAME	NUM_EMPLOYEE	AVG_SALARY
Accounting	1	8000
Sales	2	8000

SQL - Wikitechy

Can you use UNIQUE KEY as a replacement for Primary Key ?

What are all the different attributes of indexes ?

Access Types:

Access Time:

Insertion Time:

Deletion Time:

Space Overhead:

What are the different types of SQL commands ?

DDL(Data Definition Language)

Commands in DDL are

Data Manipulation Language

Data Control Language

Transaction Control Language

Difference between SQL and PLSQL

Char vs Varchar in SQL

Char

VarChar

What is Default constraint in SQL ?

What are the different types of relationships in SQL ?

Different Types of Relational DataBase Tables

One to One Relationship

One to many Relationship

Many to Many Relationship

Types of indexes in SQL

Clustered Index

Non Clustered index

Unique Index

Filtered Index

Column Store Index

Hash Index

What is the difference between BETWEEN and IN operators in SQL ?

What is a join in SQL ? What are the types of joins ?

Inner Join

Left Join

Right Join or Right Outer Join

Full Join or Full Outer Join

Cross Join

Self Join

What are the different types of database management systems ?

Hierarchical Database

Network Database

Relational Database

Object-Oriented Database

What is a Database ?

What is a primary key ?

What is Normalization in SQL ?

What is SQL ?

Lazy writer in Memory ?

Lazy write

Lazy writer in Memory

What is the difference between the SubQuery and Corelated SubQuery ?

Subquery

Correlated SubQuery

What is the Difference between join and union ?

Union

Syntax

Join

Syntax

What is the SQLCode -501 ?

What is the SQLcode -501 ?

What is the difference between a HAVING CLAUSE and a WHERE CLAUSE ?

Example of WHERE and HAVING clause

SELECT * FROM Wikitechy_Employee;

SELECT * FROM Department;

Sample Code

Output

Sample Code

Output