Prepared Statements

Under the hood, whenever you ask next.jdbc to execute some SQL it creates a java.sql.PreparedStatement, adds in the parameters you provide, and then calls .execute on it. Then it attempts to get a ResultSet from that and either return it or process it. If you asked for generated keys to be returned, that ResultSet will contain the those generated keys if your database supports it, otherwise it will be whatever the .execute function produces. If no ResultSet is available at all, next.jdbc will ask for the count of updated rows and return that as if it were a result set.

If you have a SQL operation that you intend to run multiple times on the same java.sql.Connection, it may be worth creating the prepared statement yourself and reusing it. next.jdbc/prepare accepts a connection and a vector of SQL and optional parameters and returns a java.sql.PreparedStatement which can be passed to reducible!, execute!, or execute-one! as the first argument. It is your responsibility to close the prepared statement after it has been used.

If you need to pass an option map to reducible!, execute!, or execute-one! when passing a prepared statement, you must pass nil or [] as the second argument:

(with-open [con (jdbc/get-connection ds)]
  (with-open [ps (jdbc/prepare con ["..." ...])]
    (execute-one! ps nil {...})))

Prepared Statement Parameters

If parameters are provided in the vector along with the SQL statement, in the call to prepare, then set-parameter is called for each of them. This is part of the SettableParameter protocol:

• (set-parameter v ps i) -- by default this calls (.setObject ps i v) (for nil and Object)

This can be extended to any Clojure data type, to provide a customized way to add specific types of values as parameters to any PreparedStatement. Note that you can extend this protocol via metadata so you can do it on a per-object basis if you need:

(with-meta obj {'next.jdbc.prepare/set-parameter (fn [v ps i]...)})

next.jdbc/set-parameters is available for you to call on any existing PreparedStatement to set or update the parameters that will be used when the statement is executed:

• (set-parameters ps params) -- loops over a sequence of parameter values and calls set-parameter for each one, as above.

If you need more specialized parameter handling than the protocol can provide, then you can create prepared statements explicitly, instead of letting next.jdbc do it for you, and then calling your own variant of set-parameters to install those parameters.

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