SQLExpression

protocol SQLExpression : Sendable

Overview

SQLExpressions are not well-enough organized in practice to be considered a proper Abstract Syntax Tree representation, but they nonetheless conceptually act as AST nodes. As such, anything which is executed as SQL by an SQLDatabase is represented by a value conforming to SQLExpression - even if that value is an instance of SQLRaw containing arbitrary SQL text.

The single requirement of SQLExpression is the serialize(to:) method, which must output the appropriate raw text, bindings, and/or subexpressions to the provided SQLSerializer when invoked. Most interaction with SQLDialect takes place in the serialization logic of various SQLExpressions - for example, SQLIdentifier uses the identifierQuote of the serializer’s dialect when quoting identifiers (naturally enough). Many SQLExpressions - especially those representing entire SQL queries, such as SQLSelect or SQLCreateTable - function solely as containers of other expressions which are serialized in an appropriate sequence.

See SQLSerializer and serialize(_:) for additional details regarding serialization.

Here is an example of implementing a trivial (and somewhat pointless) SQLExpression:

public struct SQLOptionalExpression<E: SQLExpression>: SQLExpression {
    public var subexpression: E?

    public init(_ subexpression: E?) {
        self.subexpression = subexpression
    }

    public func serialize(to serializer: inout SQLSerializer) {
        if let subexpression = self.subexpression {
            subexpression.serialize(to: serializer)
        }
    }
}