“ where 1 = 1”的陈述

一尘不染

“ where 1 = 1”的陈述

mysql

我看到有人使用一条语句来查询MySQL数据库中的表，如下所示：

select * from car_table where 1=1 and value="TOYOTA"

但是，这1=1是什么意思呢？

阅读 196

2020-05-17

共1个答案

一尘不染

通常是在人们建立SQL语句时。

添加时，and value = "Toyota"您不必担心之前是否存在条件或仅在哪里。优化器应该忽略它

没有魔力，只有实用

示例代码：

commandText = "select * from car_table where 1=1";

if (modelYear <> 0)     commandText += " and year="+modelYear
if (manufacturer <> "") commandText += " and value="+QuotedStr(manufacturer)
if (color <> "")        commandText += " and color="+QuotedStr(color)
if (california)         commandText += " and hasCatalytic=1"

否则，您将必须具有一组复杂的逻辑：

commandText = "select * from car_table"
whereClause = "";
if (modelYear <> 0)
{
   if (whereClause <> "") 
      whereClause = whereClause + " and ";
   commandText += "year="+modelYear;
}
if (manufacturer <> "")
{    
   if (whereClause <> "") 
      whereClause = whereClause + " and ";
   commandText += "value="+QuotedStr(manufacturer)
}
if (color <> "")
{
   if (whereClause <> "") 
      whereClause = whereClause + " and ";
   commandText += "color="+QuotedStr(color)
}
if (california)
{
   if (whereClause <> "") 
      whereClause = whereClause + " and ";
   commandText += "hasCatalytic=1"
}

if (whereClause <> "")
   commandText = commandText + "WHERE "+whereClause;

2020-05-17