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 *".
This commit is contained in:
William Carroll 2020-07-30 18:52:45 +01:00
parent dec8890190
commit 6ecab8c3a6
4 changed files with 7 additions and 7 deletions

View file

@ -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"