MySQL

FOREIGN KEY :

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:

ALTER TABLE `customer_address`  
   ADD CONSTRAINT `fk.customer_address.customer_id` 
      FOREIGN KEY (`customer_id`) REFERENCES `user` (`ID`) 
      ON UPDATE CASCADE 
      ON DELETE CASCADE;

It's very recommanded to delete unused carts after you finish the upgrade using these queries all customers carts that are not converted to order will be deleted 

select * 
FROM `quote_address`
WHERE `customer_address_id` IS NULL AND `firstname` IS NULL AND `lastname` IS NULL and quote_id in (SELECT entity_id FROM `quote` where is_active= 1)

select *    from quote where entity_id not in (select quote_id from quote_address)

we can have it like this 

SELECT A.[State], Split.a.value('.', 'VARCHAR(100)') AS String FROM (SELECT [State], CAST ('<M>' + REPLACE([City], ',', '</M><M>') + '</M>' AS XML) AS String FROM TableA) AS A CROSS APPLY String.nodes ('/M') AS Split(a);

assuiming we have table with two columns

1- State.

2- City (separated by Comma)

Please check this URL

https://stackoverflow.com/a/34207996/4135373

First of all Install Homebrew from here

https://brew.sh/

After you finish you need to run this command 

brew install percona-server

Initialize the DB

unset TMPDIR

mysqld --initialize --datadir=/usr/local/var/mysql --user={your username} --port=3307 --general-log-file=/var/log/percona/percona-error.log

Note :Don't forget to replace the username with your user {your username} = root

the Main phrase is

Select ......... where ...........  {INTO OUTFILE 'C:\\file_name.csv'}

in this wiki we have  Backing up and restoring  Database from local server and external server,

Backup Database

Backup from local server

mysqldump -u{username} -p{passoword} {database} | gzip > {path}/backup_filename_$(date +%F).sql.gz

Backup from external server

mysqldump -P {port} -h {host} -u{username} -p{passoword} {database} | gzip > {path}/backup_file_name_$(date +%F).sql.gz

Backup specific tables

mysqldump -u{username} -p{passoword} {database} table1 table2 > {path}/table1_table2.sql