Query and Exec

Learn to read rows with Query and modify with Exec.

Query and Exec

Use Query for queries that return rows (SELECT), or QueryRow for a single row; use Exec for those that don't (INSERT, UPDATE, DELETE).

rows, err := db.Query("SELECT id, ad FROM urunler")
if err != nil { return err }
defer rows.Close()
for rows.Next() {
    var id int
    var ad string
    if err := rows.Scan(&id, &ad); err != nil { return err }
}
return rows.Err()

The Exec result

Exec returns a sql.Result. RowsAffected() tells how many rows changed and LastInsertId() (on drivers that support it) the new row's ID:

res, err := db.Exec("UPDATE urunler SET stok = stok - 1 WHERE id = ?", 7)
n, _ := res.RowsAffected()
fmt.Println(n, "satır güncellendi")

For a single row, db.QueryRow(...).Scan(&x) is the short way; with no row it returns sql.ErrNoRows.

Exercise No-rows check

Try the sql.ErrNoRows check with errors.Is: declare err := sql.ErrNoRows and print errors.Is(err, sql.ErrNoRows).

Show example solution

package main

import (
    "database/sql"
    "errors"
    "fmt"
)

func main() {
    err := sql.ErrNoRows
    fmt.Println(errors.Is(err, sql.ErrNoRows))
}

Test yourself

What happens if <code>rows.Close()</code> is forgotten?
- The query runs twice
- It has no effect
- The data is deleted
- The connection isn't returned to the pool, which can run dry
Show answer

The connection isn't returned to the pool, which can run dry — Open Rows hold a connection; if you don't finish the loop or call Close, connections leak.

Read the rows

for rows.____() {
    rows.Scan(&id)
}

Show answer

for rows.Next() {
    rows.Scan(&id)
}

Which method?

SELECT many rows

Query
SELECT one row

QueryRow
INSERT / UPDATE

Exec
No-rows error

sql.ErrNoRows

Next lesson: Prepared statement

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