Queries

Composing queries

Query is a Monoid, and each piece carries its own parameters. When pieces are joined, their placeholders are renumbered, so a query built from optional parts refers to the right values however it was assembled:

let conds = catMaybes
      [ (\d -> "birth_date < " <> param d) <$> maybeCutoff
      , (\t -> "name LIKE " <> param t) <$> maybePattern
      ]
    whereClause | null conds = mempty
                | otherwise  = " WHERE " <> mconcat (intersperse " AND " conds)
rows <- query conn ("SELECT id, name, email, birth_date FROM dbo.customer" <> whereClause
                     <> " ORDER BY " <> escapeIdentifier sortColumn)

Names from data#

A column or table name that comes from data goes through escapeIdentifier, which bracket-quotes it as [name].

Text as it is#

rawSql splices text in as it is. Use it for SQL that is genuinely text rather than a value. A string literal is the same thing: a query with no parameters.

Logging#

queryText renders a query as it will be sent, with the values replaced by @p1, @p2 and so on. show does the same, and never includes a value.

How it works#

A Query is a tree of pieces, and each piece numbers its own placeholders. The absolute numbers are computed once, when the query is sent, so joining pieces is cheap however many there are.