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authorWilliam Carroll <wpcarro@gmail.com>2020-07-30T17·52+0100
committerWilliam Carroll <wpcarro@gmail.com>2020-07-30T17·52+0100
commit6ecab8c3a625d58fa5c4c5daf62a6567b4fc7701 (patch)
tree0fe10c6750640706b055fa64e78e92f34dbde392 /src/Trips.hs
parentdec8890190ff0b86f1a50044814701ef39b808e6 (diff)
Prefer SELECT (a,b,c) to SELECT *
"SELECT *" in SQL may not guarantee the order in which a record's columns are
returned. For example, in my FromRow instances for Account, I make successive call

The following scenario silently and erroneously assigns:

firstName, lastName = lastName, firstName

```sql
CREATE TABLE People (
  firstName TEXT NOT NULL,
  lastName TEXT NOT NULL,
  age INTEGER NOT NULL,
  PRIMARY KEY (firstName, lastName)
)
```

```haskell
data Person = Person { firstName :: String, lastName :: String, age :: Integer }

fromRow = do
  firstName <- field
  lastName  <- field
  age       <- field
  pure Person{..}

getPeople :: Connection -> IO [Person]
getPeople conn = query conn "SELECT * FROM People"
```

This silently fails because both firstName and lastName are Strings, and so the
FromRow Person instance type-checks, but you should expect to receive a list of
names like "Wallace William" instead of "William Wallace".

The following won't break the type-checker, but will result in a runtime parsing
error:

```haskell
-- all code from the previous example remains the same except for:

fromRow = do
  age       <- field
  firstName <- field
  lastName  <- field
```

The "SELECT *" will return records like (firstName,lastName,age), but the
FromRow instance for Person will attempt to parse firstName as
Integer.

So... what have we learned? Prefer "SELECT (firstName,lastName,age)" instead of
"SELECT *".
Diffstat (limited to 'src/Trips.hs')
-rw-r--r--src/Trips.hs2
1 files changed, 1 insertions, 1 deletions
diff --git a/src/Trips.hs b/src/Trips.hs
index 0d805cbe8603..a7880b503ddd 100644
--- a/src/Trips.hs
+++ b/src/Trips.hs
@@ -24,4 +24,4 @@ delete dbFile tripPK =
 -- | Return a list of all of the trips in `dbFile`.
 list :: FilePath -> IO [T.Trip]
 list dbFile = withConnection dbFile $ \conn ->
-  query_ conn "SELECT * FROM Trips"
+  query_ conn "SELECT (username,destination,startDate,endDate,comment) FROM Trips"