在ActiveRecord中（使用Ruby on Rails），如果@products变量@products = Product.all，我说：
@products.where("name = 'check123' ")，它返回一个符合该条件的对象数组，如果我去@products.where('name="check123"')我得到一个错误？
: SELECT "products".* FROM "products" WHERE (name = "check123")
Hirb Error: PG::UndefinedColumn: ERROR: column "check123" does not exist
LINE 1: SELECT "products".* FROM "products" WHERE (name = "check123"...
^
为什么会这样？似乎我必须总是在where子句中使用双引号，并在那里使用任何字符串的单引号？
不应该单引号在这里工作，或者有什么东西我没有得到
其他一些观察结果：
@products.where("cost = '23.0'")有效，虽然23的数据类型为整数而不是字符串？
@products.where('cost = 23')有效，所以我知道我可以在where子句
注意：我知道使用'？' where子句中的语法，以避免sql注入，我故意尝试执行这样的查询。
答案 0 :(得分：2)
双引号用于命名数据库对象（表名，列名，用户名，模式名......），而单引号用于表示要用作值的字符串。
所以您的public class NewClass {
public static void main(String[] args) {
PairOfDice player1 = new PairOfDice();
PairOfDice player2 = new PairOfDice();
Scanner scan = new Scanner(System.in);
int p1 = 33, p2 = 0, turnp1 = 0, turnp2 = 0, signal = 1;
while (p1 <= 100 || p2 >= 100) {
int newp1total = p1;
turnp1 = 0;
while (turnp1 <= 20 && signal == 1) {
System.out.println("Player 1s Turn!");
int die1 = player1.rollOne();
int die2 = player1.rollTwo();
int sumofdice = player1.sumOfRoll();
System.out.println("Player 1: First Die: " + die1 + " Second Die:" + die2 + " Sum of Roll: " + sumofdice);
if (sumofdice == 2) {
p1 = 0;
turnp1 = 0;
signal = -1;
System.out.println("Player rolled a two 1s. All players points are forfeited. Next Players turn.");
System.out.println("Points this turn:" + turnp1);
System.out.println("Points this game: " + p1);
System.out.println();
} else if (die1 == 1 || die2 == 1) {
turnp1 = 0;
signal = -1;
p1 = newp1total;
System.out.println("Player rolled a 1. All points on this round are forfeited. Next Players turn.");
System.out.println("Points this turn:" + turnp1);
System.out.println("Points this game: " + p1);
System.out.println();
} else {
turnp1 += sumofdice;
p1 += sumofdice;
System.out.println("Points this turn:" + turnp1);
System.out.println("Points this game: " + p1);
System.out.println();
signal = 1;
}
}
signal = 1;
String yesno = "y";
int newp2total = p2;
while (yesno.toLowerCase().equals("y") && signal == 1) {
System.out.println("Player 2s Turn!");
int die1 = player2.rollOne();
int die2 = player2.rollTwo();
int sumofdice = player2.sumOfRoll();
System.out.println("Player 2: First Die: " + die1 + " Second Die:" + die2 + " Sum of Roll: " + sumofdice);
if (sumofdice == 2) {
p2 = 0;
turnp2 = 0;
signal = -1;
System.out.println("Player rolled a two 1s. All players points are forfeited. Next Players turn.");
System.out.println("Points this turn:" + turnp2);
System.out.println("Points this game: " + p2);
System.out.println();
} else if (die1 == 1 || die2 == 1) {
signal = -1;
turnp2 = 0;
p2 = newp2total;
System.out.println("Player rolled a 1. All points on this round are forfeited. Next Players turn.");
System.out.println("Points this turn:" + turnp2);
System.out.println("Points this game: " + p2);
System.out.println();
} else {
turnp2 += sumofdice;
p2 += sumofdice;
System.out.println("Points this turn:" + turnp2);
System.out.println("Points this game: " + p2);
System.out.println();
System.out.println("Try your luck? Y/N");
yesno = scan.next();
System.out.println();
signal = 1;
}
}
}
}
}
有意义，因为使用UndefinedColumn: ERROR过滤生成的SQL将为@products.where('name="check123"')。数据库引擎正在尝试查找名为.... where name = "check123"的列，并将其值与列check123的值相匹配。由于您的表中不存在列name，因此您会得到一个＆＃34;未定义的列名称＆＃34;错误。
对于您的其他问题：
当您按字符串值过滤整数列时，会发生的情况是，db引擎会将列值隐式转换为字符串以执行搜索。
this question's answers描述了一般的SQL标准。总之，大多数Db引擎遵循大多数ANSI标准，并且通常为数据库对象保留双引号的使用。