SQL: Where 1=1 and 1=2

数据科学 SQL
#1

In SQL, would you mind explaining the following:

when to use where 1=1, 
when to use where 1=2, 
when to use where 1=1 and 1=2?

It would be great to explain with some examples. Thank you!

#2
  1. WHERE 1=1 表示always true, 有时会用于SQL代码生成:
base_query = "SELECT * FROM table_name WHERE 1=1 "
if (some_condition)
    base_query += "AND column1 = value1 "
if (another_condition)
    base_query += "AND column2 = value2 "

用WHERE 1=1 ，你可以用AND 添加其他条件，不需要检查它是否是第一个条件。

  1. WHERE 1=2, always false:
SELECT * FROM table_name WHERE 1=2;

这个query不会返回任何数据，但会给column names aka metadata.经常用于把table structure copy给另一个table

  1. where 1=1 and 1=2 感觉一般不会这么写…
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