Adds multirow insert method

[NeedleInAJayStack]

This adds functionality to do a multi-row inserts with a single values method call.

Previously, to insert multiple rows a user had to call values repeatedly:

db.insert(into: "planets")
    .columns(["name", "color"])
    .values([SQLBind("Jupiter"), SQLBind("orange")])
    .values([SQLBind("Mars"), SQLBind("red")])
    .run()

This was a bit awkward when inserting rows from an array, where an instance of the builder had to be saved off and edited:

let rows: [[SQLExpression]]  = [[...], [...], ...]
let builder = db.insert(into: "planets")
    .columns(["name", "color"])
for row in rows {
    builder.values(row)
}
builder.run()

This MR simplifies the mutli-row insert situation by adding a values method overload that accepts a nested array:

db.insert(into: "planets")
    .columns(["name", "color"])
    .values([[SQLBind("Jupiter"), SQLBind("orange")], [SQLBind("Mars"), SQLBind("red")]])
    .run()

let rows  = [[...], [...], ...]
db.insert(into: "planets")
    .columns(["name", "color"])
    .values(rows)
    .run()

NOTE

This functionality was only added to the SQLExpression version of values, NOT the Encodable version of values. There are known issues with [Encodable] conforming to Encodable that prevent adequate type checks.

For example, if a values(_ rows: [[Encodable]]) is defined, it is undeterministic since the same call would also match values(_ rows: [Encodable]). If instead we change values(_ rows: [Encodable]) to detect [[Encodable]] cases at runtime, then it is difficult to guarantee that a caller hasn't mixed Encodable and [Encodable] values.

[codecov-commenter]

Codecov Report

Merging #153 (c5b1f51) into main (a5c3df4) will increase coverage by 0.00%. The diff coverage is 100.00%.

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Additional details and impacted files

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##             main     #153   +/-   ##
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  Coverage   69.27%   69.28%           
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  Files          94       94           
  Lines        3486     3487    +1     
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+ Hits         2415     2416    +1     
  Misses       1071     1071           

Flag	Coverage Δ
unittests	69.28% <100.00%> (+<0.01%)	:arrow_up:

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Impacted Files	Coverage Δ
Sources/SQLKit/Builders/SQLInsertBuilder.swift	81.18% <100.00%> (+0.77%)	:arrow_up:
Sources/SQLKit/Query/SQLQueryString.swift	89.65% <0.00%> (-0.51%)	:arrow_down:

[NeedleInAJayStack]

welcome a bit of bikeshedding on the matter

I think multiValues(_:) is a fine name. Along the same vein, allValues seems like a reasonable option. However, I think I prefer rows(_:) or rowValues(_:), since I feel like they imply that all sequences should have equal length and correspond to the column ordering.

[NeedleInAJayStack]

@gwynne Sorry it took so long to get back to this one. I've adjusted it to use your implementation (which is great!), and named it rows. Again, I'm happy to switch to multiValues if you prefer that. Thanks!