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table images : { Id : int, Content : blob }
table tags : { Id : int, Tag : string }
datatype mode = Present | Absent
type condition = { Tag : string, Mode : mode }
type tag_query = sql_query [] [] [] [Id = int]
fun addCondition (c : condition) (q : tag_query) : tag_query =
case c.Mode of
Present => (SELECT I.Id AS Id
FROM ({{q}}) AS I
JOIN tags ON tags.Id = I.Id AND tags.Tag = {[c.Tag]})
| Absent => (SELECT I.Id AS Id
FROM ({{q}}) AS I
LEFT JOIN tags ON tags.Id = I.Id AND tags.Tag = {[c.Tag]}
WHERE tags.Tag IS NULL)
fun withConditions (cs : list condition) : tag_query =
List.foldl addCondition (SELECT images.Id AS Id FROM images) cs
fun main (cs : list condition) : transaction page =
x <- queryX (withConditions cs) (fn r => <xml><li>{[r.Id]}</li></xml>);
return <xml><body>
{x}
</body></xml>
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