Opaleye.Order

Ordering, LIMIT and OFFSET

Order by

orderBy :: Order a -> Select a -> Select a

Order the rows of a Select according to the Order.

import Data.Monoid ((<>))

-- Order by the first column ascending.  When first columns are equal
-- order by second column descending.
example :: Select (Column SqlInt4, Column SqlText)
        -> Select (Column SqlInt4, Column SqlText)
example = orderBy (asc fst <> desc snd)

data Order a

An Order a represents a sort order and direction for the elements of the type a. Multiple Orders can be composed with mappend or (<>) from Data.Monoid. If two rows are equal according to the first Order in the mappend, the second is used, and so on.

Instances

Divisible Order

Methods

divide :: (a -> (b, c)) -> Order b -> Order c -> Order a

conquer :: Order a

Decidable Order

Methods

lose :: (a -> Void) -> Order a

choose :: (a -> Either b c) -> Order b -> Order c -> Order a

Contravariant Order

Methods

contramap :: (a -> b) -> Order b -> Order a

(>$) :: b -> Order b -> Order a

Semigroup (Order a)

Methods

(<>) :: Order a -> Order a -> Order a

sconcat :: NonEmpty (Order a) -> Order a

stimes :: Integral b => b -> Order a -> Order a

Monoid (Order a)

Methods

mempty :: Order a

mappend :: Order a -> Order a -> Order a

mconcat :: [Order a] -> Order a

Order direction

asc :: SqlOrd b => (a -> Column b) -> Order a

Specify an ascending ordering by the given expression. (Any NULLs appear last)

desc :: SqlOrd b => (a -> Column b) -> Order a

Specify an descending ordering by the given expression. (Any NULLs appear first)

ascNullsFirst :: SqlOrd b => (a -> Column b) -> Order a

Specify an ascending ordering by the given expression. (Any NULLs appear first)

descNullsLast :: SqlOrd b => (a -> Column b) -> Order a

Specify an descending ordering by the given expression. (Any NULLs appear last)

Limit and offset

limit :: Int -> Select a -> Select a

Limit the results of the given query to the given maximum number of items.

WARNING: If you're planning on using limit/offset together please use offset before you use limit, e.g.:

limit 10 (offset 50 yourQuery)

This is because Opaleye applies OFFSET and LIMIT to the query separately. The result of the query given above is the following, which will return 10 rows after skipping the first 50 (probably what you want).

SELECT * FROM (SELECT * FROM yourTable OFFSET 50) LIMIT 10

However, reversing the order of the limit/offset will result in the following, which will result in no rows being returned (probably not what you want).

SELECT * FROM (SELECT * FROM yourTable LIMIT 10) OFFSET 50

offset :: Int -> Select a -> Select a

Offset the results of the given query by the given amount, skipping that many result rows.

WARNING: Please read the documentation of limit before combining offset with limit.

Exact ordering

exact :: [Column b] -> (a -> Column b) -> Order a

Order the results of a given query exactly, as determined by the given list of input columns. Note that this list does not have to contain an entry for every result in your query: you may exactly order only a subset of results, if you wish. Rows that are not ordered according to the input list are returned after the ordered results, in the usual order the database would return them (e.g. sorted by primary key). Exactly-ordered results always come first in a result set. Entries in the input list that are not present in result of a query are ignored.

Other

class PGOrd a

Typeclass for Postgres types which support ordering operations.

Instances

PGOrd SqlCitext

PGOrd SqlUuid

PGOrd SqlTimestamptz

PGOrd SqlTimestamp

PGOrd SqlTime

PGOrd SqlText

PGOrd SqlNumeric

PGOrd SqlInt2

PGOrd SqlInt4

PGOrd SqlInt8

PGOrd SqlFloat8

PGOrd SqlFloat4

PGOrd SqlDate

PGOrd SqlBool

PGOrd a => PGOrd (Nullable a)

type SqlOrd = PGOrd