Add SQLLAGExpression

[ptoffy]

This adds support for the LAG window function which allows support for accessing a value from a previous row

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.86%. Comparing base (785f0bb) to head (052e899).

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
+ Coverage   91.68%   91.86%   +0.18%     
==========================================
  Files          61       63       +2     
  Lines         926      947      +21     
==========================================
+ Hits          849      870      +21     
  Misses         77       77              

Files with missing lines	Coverage Δ
...LKitExtras/Expressions/SQLLAG+FluentKeypaths.swift	100.00% <100.00%> (ø)
...as/SQLKitExtras/Expressions/SQLLAGExpression.swift	100.00% <100.00%> (ø)

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[gwynne]

The comment on the serialize method is purely advisory; I would really prefer to add syntax that provides more general support for window functions rather than for just one specific function. Indeed, even this one function's implementation is incomplete without at least adding support support for PARTITION BY and FILTER and correct support for ORDER BY. The correct syntax of lag() is lag(value[, offset[, default]]), per https://www.postgresql.org/docs/17/functions-window.html, https://dev.mysql.com/doc/refman/9.0/en/window-function-descriptions.html#function_lag, and https://sqlite.org/windowfunctions.html#built_in_window_functions - where you got this OVER (ORDER BY 1) DEFAULT 0 syntax I can't even guess.

[gwynne]

It's always preferable to leverage existing expressions when possible in generating output for new ones - and to use serializer.statement {} when the syntax allows. For example:

Suggested change

	serializer.write("LAG(")
	value.serialize(to: &serializer)
	serializer.write(") OVER (ORDER BY ")
	offset.serialize(to: &serializer)
	serializer.write(")")
	if let defaultValue = defaultValue {
	serializer.write(" DEFAULT ")
	defaultValue.serialize(to: &serializer)
	}
	serializer.statement {
	$0.append(SQLFunction("lag", args: value))
	$0.append("OVER")
	$0.append(SQLGroupExpression(SQLList([SQLRaw("ORDER BY"), offset], separator: SQLRaw(" ")))
	if let defaultValue = self.defaultValue {
	$0.append(SQLLiteral.default, defaultValue)
	}
	}

(keeping in mind that this is not even correct syntax for LAG to begin with - see the overall review comment for more details)

[ptoffy]

@gwynne do you want to add proper window function support? I think that should be doable. As for the LAG syntax, what I added is what I've successfully been using in a production system for some time now - I got it from an example somewhere (I think it was https://www.geeksforgeeks.org/postgresql-lag-function/ but I'm not 100% sure)

[gwynne]

I definitely want to add proper support, if we're gonna add anything at all.

Also, that link also shows correct syntax. I think what you have mostly works by accident 😅