问题描述

可能的重复:
为什么有人会使用 WHERE 1=1 AND <条件>在 SQL 子句中?

我看到有人用语句查询 MySQL 数据库中的表，如下所示:

I saw some people use a statement to query a table in a MySQL database like the following:

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

但是1=1在这里是什么意思?

But what does 1=1 mean here?

推荐答案

通常是人们构建 SQL 语句的时候.

It's usually when folks build up SQL statements.

当您添加和value = "Toyota"时，您不必担心是否有条件之前或只是WHERE.优化器应该忽略它

When you add and value = "Toyota" you don't have to worry about whether there is a condition before or just WHERE. The optimiser should ignore it

没有魔法，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"

否则你将不得不有一套复杂的逻辑:

Otherwise you would have to have a complicated set of logic:

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;

这篇关于“其中 1=1"陈述的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持编程学习网！