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http://www.luciopanasci.it/Ebooks/MySQL%20Cookbook,%203rd%20Edition.pdf [In depth]
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https://downloads.mysql.com/docs/refman-8.0-en.pdf [Direct from MYSQL itself]
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https://www.hackerrank.com/domains/databases [Practice problems]
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http://iips.icci.edu.iq/images/exam/databases-ramaz.pdf [How to design a database]