Haskell，HDBC.Sqlite3-如果尚不存在该如何添加列？

Question

I have a function that given an Int returns a list of lists of Strings.

fetchParts :: Int -> [[String]]

This is what the output looks like

[["title", "some title"], ["rate", "2.4"], ["dist", "some string"], ["tr", "1"], ["td, "2"] ..]]

The length of the output can be variable. Only the first 3 lists can be present 100% of the time.

The later part of the list can be

["a", "1"], ["b", "2"] ..

or

["some", "1"], ["part", "2"], ["of", "3"] ..]

or

["ex1", "a"], ["ex2", "b"], ..]

or some other combination of strings.

And I want to add this output to a sqlite3 database file. I'm using HDBC and HDBC.Sqlite3 for this.

To add something to a database file I'm running functions like these

  initialConnection <- connectSqlite3 "src/parts.db"
  run initialConnection partsEntry []
  commit initialConnection
  disconnect initialConnection

where partsEntry is a simple SQL String like this

partsEntry = "INSERT INTO PARTSDATA ( title, rate, dist, ...) VALUES ( "some title", "2.4", "some string", ...)

where

( title, rate, dist, ...) are from head <$> fetchParts 1

and

("some title", "2.4", "some string" ...) are from last <$> fetchParts 1

The problem is say if "some" column doesn't exists, code will throw errors.

What I want to do is something like this

• if column "abc" doesn't exists, add column "abc" and insert "this" value at the current row
• if column "abc" exists, just insert "this" value at the current row

But I'm not sure how to go about doing that.

Answer 1

I was able to solve the problem.

First use describeTable function from HDBC package. The function will return column names and type. If you just need the names like I did, this is what you can do

getColumnsInTable :: conn -> String -> IO [String]
getColumnsInTable conn tableName = do
  d <- describeTable conn tableName
  return $ fst <$> d

The return will have all the columns' names.

Scan through the list to see if it contains all the columns you wish. If it doesn't use a function like the following to alter the table, ie add a new column with INT type.

createNewColumn conn columnName = do
  let stmt = "ALTER TABLE FantasyBooks ADD COLUMN " ++ columnName ++ " INT;"
  run conn stmt []