sed -n 5p <file>

- To print a specific line from a file

sed -n '10,20p' <filename>
- Prints all the lines between 10 and 20 of a file
awk 'NR >= 3 && NR <= 6' /path/to/file
- Prints all lines between two line numbers
sed -n '3,6p' /path/to/file
- Prints all lines between two line numbers
sed '10,20!d' <filename>
- Prints all the lines between 10 and 20 of a file
awk 'FNR==5' <file>
- To print a specific line from a file
sed '1d' file-name
-Delete 1
st
line of a file
sed '10d' file-name
-Delete 10
th
line of a file
sed '5,10d' <file-name>
-Delete lines 5 to 10 in a file
sed '2,$d' filename
-deletes all lines except the first line in a file which can also be done with sed '1!d' and sed -n '1p' filename.

awk {print $0} <filename>
-To print all the fields/columns in a file.
awk {print $3} <filename>
-To print only the 3
rd
field/column of a file.
awk {print $3 $5} <filename>
-To print the 3
rd
and 5
th
fields/columns of a file
awk F , {print $0} <filename>
-To print all the fields/columns of a file with , as the separator
awk F , {print $3} <filename>
-To print the 3
rd
field/column of a file with , as the separator
awk F : {print $5} <filename>
-To print the 5
th
field/column of a file with , as the separator
awk {$3=; print $0} <filename>
-To print all the fields/columns except the 3
rd
one of a file.
awk {$2=$3=; print $0} <filename>
-To print all the fields/columns except the 2
nd
and 3
rd
of a file.

Deleting the blank lines from a file:
Using grep:
grep v ^$ <filename>
Using sed:
sed /^$/d <filename>
Using awk:
awk /./ <filename>
awk {print add_to_beginning$0} <filename> Add text at the beginning of each line
sed s/^/add_to_beginning/ <filename> Add text at the beginning of each line.
awk {print #$0} <filename> Comment out all lines
sed s/^/#/ <filename> Comment out all lines.
awk {print $0append_to_end} <filename> Add text at the end of each line
sed s/$/append_to_end/ <filename> Add text at the end of each line
awk {print $0;} <filename> Add ; at the end of each line
awk {print add_to_beginning$0add_to_end} <filename> Add text at the beginning and end
of each line.
sed s/.*/add_to_beginning&add_to_end/ <filename> Add text at the beginning and end of
each line.

SQL Joins:
INNER JOIN: Returns all rows when there is at least one match in BOTH tables
LEFT JOIN: Return all rows from the left table, and the matched rows from the right table
RIGHT JOIN: Return all rows from the right table, and the matched rows from the left table
FULL JOIN: Return all rows when there is a match in ONE of the tables.
SQL Constraints:
NOT NULL - Indicates that a column cannot store NULL value
UNIQUE - Ensures that each row for a column must have a unique value
PRIMARY KEY - A combination of a NOT NULL and UNIQUE. Ensures that a column (or
combination of two or more columns) have an unique identity which helps to find a
particular record in a table more easily and quickly
FOREIGN KEY - Ensure the referential integrity of the data in one table to match values in
another table
CHECK - Ensures that the value in a column meets a specific condition
DEFAULT - Specifies a default value when specified none for this column.
PRIMARY KEY Constraints:
1. The PRIMARY KEY constraint uniquely identifies each record in a database table.
2. Primary keys must contain unique values.
3. A primary key column cannot contain NULL values.
4. Each table should have a primary key, and each table can have only ONE primary key.
VIEWS:
A view is a virtual table.
CREATE VIEW Statement:
1. In SQL, a view is a virtual table based on the result-set of an SQL statement.
2. A view contains rows and columns, just like a real table. The fields in a view are fields from
one or more real tables in the database.
SQL CREATE VIEW Syntax:
CREATE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition

SQL Updating a View:
You can update a view by using the following syntax:
SQL CREATE OR REPLACE VIEW Syntax:
CREATE OR REPLACE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition

SQL Dropping a View:
You can delete a view with the DROP VIEW command.
DROP VIEW view_name.
SQL Aggregate Functions:
SQL aggregate functions return a single value, calculated from values in a column.
Useful aggregate functions:
AVG() - Returns the average value
COUNT() - Returns the number of rows
FIRST() - Returns the first value
LAST() - Returns the last value
MAX() - Returns the largest value
MIN() - Returns the smallest value
SUM() - Returns the sum
SQL Scalar functions:
SQL scalar functions return a single value, based on the input value.
Useful scalar functions:
UCASE() - Converts a field to upper case
LCASE() - Converts a field to lower case
MID() - Extract characters from a text field
LEN() - Returns the length of a text field
ROUND() - Rounds a numeric field to the number of decimals specified
NOW() - Returns the current system date and time
FORMAT() - Formats how a field is to be displayed.