Are you still facing the issue? I might be able to help you out.
Here are some more reasons why it might be happening:
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A corrupted database – Bad plugins, addons, themes and data-transfer interruptions might leave your database short of tables or completely corrupted at worst.
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Corrupted WordPress files
Found this on: http://www.wpkube.com/how-to-fix-error-establishing-database-connection-in-wordpress/