README for SQL code

Author: tobyhede

sql/README.md

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+### Adding SQL
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+- Never drop the configuration table as it may contain customer data and needs to live across EQL versions
+- Everything else should have a DROP IF EXISTS
+- Functions should be DROP and CREATE, instead of CREATE OR REPLACE
+ - Data types cannot be changed once created, so dropping first is more flexible
+- Keep DROP and CREATE together in the code
+- Types need to be dropped last, add to the `666-drop_types.sql`
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