我可以从同一列中选择多个值吗答案

【问题标题】：Can I SELECT more than one value from the same column我可以从同一列中选择多个值吗
【发布时间】：2020-07-07 19:50:08
【问题描述】：

我正在尝试使用单个 MySql.Data SELECT 语句从不止一行中获取值。

我尝试使用以下代码但失败了。错误说

第 # 行有语法错误

MySqlConnection mysqlcon = new MySqlConnection(M_str_sqlcon);

MySqlCommand mysqlcom = new MySqlCommand(
    "SELECT value FROM metatable WHERE(ProductID = 16 AND MetaKey = 'SKU') AND " +
    "SELECT value FROM metatable WHERE(ProductID = 16 AND MetaKey = 'Price') AND " +
    "SELECT value FROM metatable WHERE(ProductID = 16 AND MetaKey = 'Sold') AND " +
    "SELECT value FROM metatable WHERE(ProductID = 16 AND MetaKey = 'SoldDate')"
, mysqlcon);

MySqlDataReader r = mysqlcom.ExecuteReader(CommandBehavior.CloseConnection);

prodMeta.Add(new ProductMeta{
    ProductSKU = r.GetString(0),
    RegularPrice = r.GetDouble(1),
    SoldCount = r.GetInt32(2),
    SoldDate = r.GetString(3)
});

mysqlcon.Close();

【问题讨论】：

标签： c# mysql sql .net ado.net

【解决方案1】：

您可以像这样简单地使用in 运算符，而不是重复使用无效的Select 语句：

SELECT value 
FROM metatable 
WHERE ProductID = 16 AND MetaKey in ('SKU','Price', 'Sold', 'SoldDate')

而且你不需要 where 后面的括号。

【讨论】：

【解决方案2】：

您可以将查询修改为 UNION 所有输出，如下所示。 Union 结合了 2 个或多个查询的结果（确保您具有相同顺序的相似列）

MySqlConnection mysqlcon = new MySqlConnection(M_str_sqlcon);

MySqlCommand mysqlcom = new MySqlCommand(
    "SELECT value FROM metatable WHERE(ProductID = 16 AND MetaKey = 'SKU') UNION " +
    "SELECT value FROM metatable WHERE(ProductID = 16 AND MetaKey = 'Price') UNION " +
    "SELECT value FROM metatable WHERE(ProductID = 16 AND MetaKey = 'Sold') UNION " +
    "SELECT value FROM metatable WHERE(ProductID = 16 AND MetaKey = 'SoldDate')"
, mysqlcon);

MySqlDataReader r = mysqlcom.ExecuteReader(CommandBehavior.CloseConnection);

prodMeta.Add(new ProductMeta{
    ProductSKU = r.GetString(0),
    RegularPrice = r.GetDouble(1),
    SoldCount = r.GetInt32(2),
    SoldDate = r.GetString(3)
});

mysqlcon.Close();

但是，由于在此示例中您是从同一个表中提取数据，您可以使用 OR 来组合以下条件：

MySqlConnection mysqlcon = new MySqlConnection(M_str_sqlcon);

MySqlCommand mysqlcom = new MySqlCommand(
    "SELECT value FROM metatable WHERE(ProductID = 16 AND MetaKey = 'SKU')  OR (ProductID = 16 AND MetaKey = 'Price') OR (ProductID = 16 AND MetaKey = 'Sold') OR (ProductID = 16 AND MetaKey = 'SoldDate')"
, mysqlcon);

MySqlDataReader r = mysqlcom.ExecuteReader(CommandBehavior.CloseConnection);

prodMeta.Add(new ProductMeta{
    ProductSKU = r.GetString(0),
    RegularPrice = r.GetDouble(1),
    SoldCount = r.GetInt32(2),
    SoldDate = r.GetString(3)
});

mysqlcon.Close();

或者您可以使用 IN 运算符，如下所示：

MySqlConnection mysqlcon = new MySqlConnection(M_str_sqlcon);

MySqlCommand mysqlcom = new MySqlCommand(
    "SELECT value FROM metatable WHERE ProductID = 16 AND MetaKey IN ('SKU','Price','Sold','SoldDate')"
, mysqlcon);

MySqlDataReader r = mysqlcom.ExecuteReader(CommandBehavior.CloseConnection);

prodMeta.Add(new ProductMeta{
    ProductSKU = r.GetString(0),
    RegularPrice = r.GetDouble(1),
    SoldCount = r.GetInt32(2),
    SoldDate = r.GetString(3)
});

mysqlcon.Close();

【讨论】：