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A typical implementation of "multi-tenant" is to have a database for each client. Each database has the same few dozen tables. Security (if you care) is somewhat available at the database level. If y …

Both of these schema anti-patterns have been repeatedly refuted: Do not have multiple 'identical' tables. Do not spread an array across columns. So have one table with lots of rows: sites …

InnoDB tables really need an explicit PRIMARY KEY. If you don't have a 'natural' key, add an AUTO_INCREMENT. INT(3) and INT(99) mean the same thing -- a 4-byte binary number. Consider changing to T …

To focus on ongoing and future dates... A table with suitable columns for simple cases of repeated actions. One row per repeating event. No specific dates except start and end. A table for excepti …

Do NOT normalize any "continuous" or "numeric" values, such as a timestamp. First of all, TIMESTAMP and DATETIME each take 5 bytes. INT takes 4 bytes. So, that's not much savings. A 3-byte MEDIUMI …

Changing the title changes the question. FOREIGN KEYs are optional. They do: An FK implicitly create an INDEX in one of the tables. Such an index can be manually added. (So FK is not required fo …

It sounds like you have reinvented "Entity-Attribute-Value" schema. This will be painful in the long run. Meanwhile... Do not "normalize" dates or other 'continuous' values (such as FLOAT). The DA …

(Others may disagree, but here goes...) If you have a good "natural" PRIMARY KEY, use it. In my own tables, I find that is the case about 2/3 of the time. What makes it good? Must be UNIQUE (sinc …

Normalization decreases the hassles; you decide where the cutoff is -- among speed/space/hassle/etc. …

An "order" is composed of multiple "items", yes? Well, don't mix the two things in a single table. Similarly eBayOrders_account1 seems to be a mashup of orders, orderitems, users and payment. In gen …

In the table Chapter you should have a column book_id. That is the standard way to implement many:1. There no need for an extra table; that is only for many:many.

Perhaps a better policy: When registering a user, store the fee charged into the user's history. Derive the fee from some business logic that may or may not be stored in a database table. Note how …

A couple of cmments: CommentReplies seems to be implementing many-to-many. I would expect it to be one-to-many -- as in, each comment has one 'parent'. One-to-many does not need an extra table; you …

Redundant information in a database is a no-no. If you have an index (such as the PRIMARY KEY) on user_id and if there is not thousands of rows for a user, the computing it on the fly is the way to g …

Turn things around. Table addresses needs a delivery_id. Ditto for the table storage. If you want to avoid having multiple rows for addresses or storages, then have two "many-to-many" tables to map b …