英文:
sql: scan row(s) with unknown number of columns (select * from ...)
问题
我有一个包含许多列的表t，我的SQL语句是这样的：select * from t。现在我只想从返回的行集中扫描一个或两个列。然而，sql.Scan接受dest ...interface{}作为参数。这是否意味着我必须扫描所有的列，然后只使用我需要的列？
我知道我可以将SQL语句从select *改为select my_favorite_rows，但在这种情况下，我无法更改SQL语句。
英文:
I have a table t containing a lot of columns, and my sql is like this: select * from t. Now I only want to scan one column or two from the wide returned row set. However, the sql.Scan accepts dest ...interface{} as arguments. Does it mean I have to scan everything and use only the column I needed?
I know I could change the sql from select * to select my_favorite_rows, however, in this case, I have no way to change the sql.
答案1
得分: 35
你可以使用Rows.Columns来获取列名，例如：
package main
import (
"database/sql"
"fmt"
"github.com/lib/pq"
)
type Vehicle struct {
Id int
Name string
Wheels int
}
// VehicleCol返回Vehicle的列的引用
func VehicleCol(colname string, vh *Vehicle) interface{} {
switch colname {
case "id":
return &vh.Id
case "name":
return &vh.Name
case "wheels":
return &vh.Wheels
default:
panic("unknown column " + colname)
}
}
func panicOnErr(err error) {
if err != nil {
panic(err.Error())
}
}
func main() {
conn, err := pq.ParseURL("postgres://docker:docker@172.17.0.2:5432/pgsqltest?schema=public")
panicOnErr(err)
var db *sql.DB
db, err = sql.Open("postgres", conn)
panicOnErr(err)
var rows *sql.Rows
rows, err = db.Query("select * from vehicle")
panicOnErr(err)
// 从查询中获取列名
var columns []string
columns, err = rows.Columns()
panicOnErr(err)
colNum := len(columns)
all := []Vehicle{}
for rows.Next() {
vh := Vehicle{}
// 利用VehicleCol创建列的引用
cols := make([]interface{}, colNum)
for i := 0; i < colNum; i++ {
cols[i] = VehicleCol(columns[i], &vh)
}
err = rows.Scan(cols...)
panicOnErr(err)
all = append(all, vh)
}
fmt.Printf("%#v\n", all)
}
这是一个使用Go语言编写的示例代码，用于从数据库中查询车辆信息并打印出来。
英文:
You can make use of Rows.Columns, e.g.
package main
import (
"database/sql"
"fmt"
"github.com/lib/pq"
)
type Vehicle struct {
Id int
Name string
Wheels int
}
// VehicleCol returns a reference for a column of a Vehicle
func VehicleCol(colname string, vh *Vehicle) interface{} {
switch colname {
case "id":
return &vh.Id
case "name":
return &vh.Name
case "wheels":
return &vh.Wheels
default:
panic("unknown column " + colname)
}
}
func panicOnErr(err error) {
if err != nil {
panic(err.Error())
}
}
func main() {
conn, err := pq.ParseURL(`postgres://docker:docker@172.17.0.2:5432/pgsqltest?schema=public`)
panicOnErr(err)
var db *sql.DB
db, err = sql.Open("postgres", conn)
panicOnErr(err)
var rows *sql.Rows
rows, err = db.Query("select * from vehicle")
panicOnErr(err)
// get the column names from the query
var columns []string
columns, err = rows.Columns()
panicOnErr(err)
colNum := len(columns)
all := []Vehicle{}
for rows.Next() {
vh := Vehicle{}
// make references for the cols with the aid of VehicleCol
cols := make([]interface{}, colNum)
for i := 0; i < colNum; i++ {
cols[i] = VehicleCol(columns[i], &vh)
}
err = rows.Scan(cols...)
panicOnErr(err)
all = append(all, vh)
}
fmt.Printf("%#v\n", all)
}
答案2
得分: 3
对于未知列长度，但如果您对它们的类型确定，可以使用以下代码：
cols, err := rows.Columns()
if err != nil {
log.Fatal(err.Error())
}
colLen := len(cols)
vals := make([]interface{}, colLen)
for rows.Next() {
for i := 0; i < len(colLen); i++ {
vals[i] = new(string)
}
err := rows.Scan(vals...)
if err != nil {
log.Fatal(err.Error()) // 如果类型错误
}
fmt.Printf("第一列: %s\n", *(vals[0].(*string))) // 如果类型错误，将会引发 panic
}
注意：不建议在生产环境中使用此方法。
英文:
For unknown length of columns but if you're sure about their type,
cols, err := rows.Columns()
if err != nil {
log.Fatal(err.Error())
}
colLen := len(cols)
vals := make([]interface{}, colLen)
for rows.Next() {
for i := 0; i < len(colLen); i++ {
vals[i] = new(string)
}
err := rows.Scan(vals...)
if err != nil {
log.Fatal(err.Error()) // if wrong type
}
fmt.Printf("Column 1: %s\n", *(vals[0].(*string))) // will panic if wrong type
}
PS: Not recommended for prod
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