Explanations for NULL, Added Default (#28008)

1. Corrected NOT NULL / NULL explanation
2. Added explanation for Default
3. Added new columns to illustrate 'default' and 'null' behaviour.
4. Explanations for datatypes
5. NUMERIC Datatype illustrated.

guide/english/sql/sql-create-table-statement/index.md

@@ -24,17 +24,44 @@ Here’s an example creating a table named Person:
 
 ```sql
 CREATE TABLE Person(
-  Id int not null,
-  Name varchar not null,
-  DateOfBirth date not null,
-  Gender bit not null,
+  Id           int      not null,
+  Name         varchar  not null,
+  DateOfBirth  date     not null,
+  Gender       bit      not null,
+  Height       numeric(5, 2) default 0 not null, 
+  Weight       numeric(5, 2) default 50,
+  WaistSize    numeric(5, 2),
   PRIMARY KEY( Id )
 );
 ```
 
-In the example above, each Person has a Name, a Date of Birth and a Gender. The Id column is the key that identifies one person in the table. You use the keyword `PRIMARY KEY` to configure one or more columns as a primary key.
+##### PrimaryKey
+In the example above, each Person has a Name, Date of Birth,  Gender and Height. The Id column is the key that identifies one person in the table. You use the keyword `PRIMARY KEY` to configure one or more columns as a primary key.
 
-A column can be `not null` or `null` indicating whether it is mandatory or not.
+##### Datatypes
+Each column also has it's datatype mentioned. 
+
+Just to understand the above code, the datatype mentioned indicates the following type of values approximately. 
+```
+int          : A whole number
+varchar      : Strings
+date         : A Date Value without Time component
+bit          : A Boolean value
+numeric(5, 2): A decimal value with ( Precision 5 and Scale 2 ) 
+              ie. Total of 5 digits with 3 digits before the decimal point and 2 digits after decimal point.
+```
+Proper List and meaning of datatypes can be refered from resources like this: ( https://www.w3schools.com/sql/sql_datatypes.asp )
+
+##### Default and Not NULL
+When a row of values are inserted into a table, any column which is marked as `not null` and without a `default` value will be compulsary. Insert fails otherwise.
+
+If no value is provided for a column which has a `default` value, insert will succeed with the column being assigned the default value defined in the table definition.
+>e.g Column Weight  has default value as 50.
+
+By default all columns are nullable ie. `null` is assumed unless explicity set as `not null`. So any column without `default` and `not null` keyword can be skipped in insert commands. They will have *null* value on querying. 
+>e.g Column WaistSize  can be *null*
+
+>Please note that `null` and `` (Blank) are queried and handled differently in most Databases.
 
 
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