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Usage example with a context:

As we mentioned earlier, the process of changing a database design to produce table schemes in normal form is called normalization.

As a very simple example, the table scheme:

{ISBN, Title, Authors}

is not in first normal form because the Authors attribute might contain more than one author and is therefore not atomic. By trading in this table scheme for the two schemes:

{ISBN, Title, AuID} and {AuID, AuName}

we have normalized the database into first normal form.

I don't quite get how in is used in that sentence. I suppose, in this context, trading simply means exchanging. So, we exchange the previous table scheme for the two new ones, but I don't understand why we have to trade in the table to do that.

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    You can think of "trade in" as a single unit, a phrasal verb. trade in "phrasal verb [transitive] to give something old as part of the payment for something new" Commented Dec 12, 2014 at 14:25
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    I'm wondering if the writer said or meant "schema" rather than "scheme". There are "two tables" here, not "two table schemes". Though if he meant "schema" he's using the word wrong, as both tables are part of one schema. And it's not at all clear how changing a text field to a foreign key puts something into first normal form. If there are multiple authors, it is still unresolved how to record this in the schema given. But that's drifting from language to database design.
    – Jay
    Commented Dec 12, 2014 at 14:36

1 Answer 1

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Here, 'trading in' is the entire verb meaning 'an exchange' in the same way that 'trading' does. You could replace it with 'trading' without changing the meaning of the sentence, though:

By trading in this table scheme for the two schemes ... we have normalized the database into first normal form.

and

By trading this table scheme for the two schemes ... we have normalized the database into first normal form.

are the same.

'Trading in' is specifically used for contexts that involve replacing one thing for another, most commonly in selling a used car to a dealership in exchange for a discount on the purchase of another (usually newer) car.

'Trading' can refer to a wider variety of exchanges. However, when 'trading in' can be used, so can 'trading'.

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  • Small correction regarding the last sentence: You can't replace "trade in a car" with "trade a car". The former means that you're buying a car with a trade-in car plus money; the latter means that you're trading one car for another, without additional money.
    – Ben Kovitz
    Commented Dec 27, 2014 at 10:27

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