database - 如何在 haskell 中使用 HDBC 有效地检查条目是否存在？

Question

就像我在问题中提到的那样，我对要插入一些数据的表有唯一约束。我必须检查数据是否已经存在，如果不存在则将其插入。

这似乎是我做不到的。这是我的代码:

import System.Environment (getArgs)
import Database.HDBC
import Database.HDBC.Sqlite3
import Text.Regex.Posix
import qualified Data.ByteString.Char8 as B

getFrom bstr =
  bstr =~ "From:.+@.+\\.(fr|com).?" :: B.ByteString

getTo bstr =
  bstr =~ "To:.+@.+\\.(fr|com).?" :: B.ByteString

getSubject bstr =
  bstr =~ "Subject:.+" :: B.ByteString

main = do
  --[dbPath, rawMail] <- getArgs
  bstr <- B.getContents
  conn <- connectSqlite3 "bmsg.db"
  let
    sqNomDest  = toSql $ getTo bstr
    sqNomExped = toSql $ getFrom bstr
  -- begin of the problematic part

  -- I make a query to know if the entry if already present
  qdbefore <- quickQuery' conn "SELECT d_id FROM dest WHERE maildest LIKE ?" [sqNomDest]
  qebefore <- quickQuery' conn "SELECT e_id FROM exped WHERE mailexped LIKE ?" [sqNomExped]
  -- then if not I insert it and else I return an arbitrary Int
  case qdbefore of --my check is probably wrong since that snippet always go to return 0
    [[SqlNull]] -> run conn "INSERT INTO dest(maildest) VALUES(?)" [sqNomDest] --unique on the column constraint so if I ran it alone on something already present it would raise an exeption
    _           -> return 0
  case qebefore of
    [[SqlNull]] -> run conn "INSERT INTO exped(mailexped) VALUES(?)" [sqNomExped] --same here for the constraint
    _           -> return 0
  commit conn
  --end of the problematic part

  [[qd]] <- quickQuery' conn "SELECT d_id FROM dest WHERE maildest LIKE ?" [sqNomDest]
  [[qe]] <- quickQuery' conn "SELECT e_id FROM exped WHERE mailexped LIKE ?" [sqNomExped]
  run conn "INSERT INTO mails(d_id, e_id, sujet, mail) VALUES (?, ?, ?, ?)" [qd, qe, toSql $ getSubject bstr, toSql bstr]
  commit conn
  disconnect conn


-- CREATE TABLE dest
-- (
-- d_id INTEGER PRIMARY KEY,
-- maildest VARCHAR(64) NOT NULL UNIQUE
-- );

-- CREATE TABLE exped
-- (
-- e_id INTEGER PRIMARY KEY,
-- mailexped VARCHAR(64) NOT NULL UNIQUE
-- );

-- CREATE TABLE mails
-- (
-- m_id    INTEGER PRIMARY KEY,
-- d_id    INTEGER NOT NULL,
-- e_id    INTEGER NOT NULL,
-- sujet   VARCHAR(128),
-- mail    TEXT NOT NULL,
-- CONSTRAINT fk_dest FOREIGN KEY (d_id) REFERENCES dest(d_id),
-- CONSTRAINT fk_exped FOREIGN KEY (e_id) REFERENCES exped(e_id)
-- );

没有错误，它编译了，但是当我给它一个不存在的电子邮件时模式匹配失败。

Answer 1

如果您的查询没有结果，您将得到一个空列表，而不是一个包含一行和一个空的列表——这就是为什么当您插入一个新事物时 case 失败的原因。对于失败的情况，只匹配一个空列表。