For what reasons can data fail to be retrieved from a database?

Data may not be retrieved from the database for several reasons:

• Query errors: syntax errors, incorrect WHERE conditions, missing JOINs.
• No data: there are simply no records in the table that satisfy the query.
• Connection issues: database is unavailable, timeouts, network errors.
• Access rights: the user does not have permission to read the required tables.
• Transaction problems: data is locked or not committed.
• Incorrect result handling: errors in code, incorrect row scanning.

Example in Go with error checking during data retrieval:

row := db.QueryRow("SELECT name FROM users WHERE id = ?", userID)
var name string
err := row.Scan(&name)
if err == sql.ErrNoRows {
    fmt.Println("User not found")
} else if err != nil {
    log.Fatal(err)
} else {
    fmt.Println("User name:", name)
}