CODEWITHSUNDEEP
sqlgo
Adam Kalnas
Adam Kalnas

Reputation: 1198

How do I get the number of rows returned while using database/sql?

Given the following function:

func (me *OrderService) GetOrders(orderTx *sql.Tx, orderId int) (orders *sql.Rows) {
    orders, err := ecommTx.Query("SELECT * FROM orders WHERE id=?", orderId)
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("Successfully queried and receive %d orders", orders.count)
    return orders
}

Are there any easy ways to .count the results? I'd like to keep this database engine agonistic, but FWIW.... I'm using Matt N's sqlite3 driver for my integration tests, but plan on having a different DB in prod.

Upvotes: 1

Views: 2186

Answers (2)

Vector
Vector

Reputation: 11703

sql.Query() returns *Rows. Rows is a reader, not a collection, so calls such as count() or len() etc. are not relevant. You have to read through Rows to know how many entries you've got: 

count := 0
for orders.Next() {
    count++
}

log.Printf("Successfully queried and receive %d orders", count)

You are not adding any extra overhead by doing so. Other languages, such as C# or Delphi, that might return a collection with a count property are simply doing the reading for you and packaging the results into a collection for your convenience.

Upvotes: 2

Damien Tournoud
Damien Tournoud

Reputation: 586

There is no portable way to know the number of rows returned by a statement in advance, meaning without iterating through the returned cursor and counting.

There are two typical solutions to this:

  • Run a separate COUNT(*) query. The problem with this approach is that it's very racy (the result set can be modified between the two queries, at least in some transaction isolation modes), so it is really only useful for things like pagers where you do a query that returns only part of the result set, and want to know how many other rows there are.

  • Iterate through the cursor and count. In Go, that means calling .Next() and then .Scan the result into your Order struct.

Upvotes: 3

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