Deep understanding of where 1=1 is useful _mysql

What's the use of where 1=1? In the SQL language, writing such a sentence is not the same as writing.

The select * FROM table1 where 1=1 is completely indistinguishable from the select * from table1, and there are even many other written, 1<>2, ' a ' = ' a ', ' a ' <> ' B ', whose purpose is only one, where the condition is forever, the result is the unconstrained condition.

This is used in SQL injection, such as the SELECT * from table1 where name= ' lala ' to forcibly add a select * from table1 where name= ' lala ' or 1=1 this becomes an unconstrained query.

The recent discovery of the magic is that, in the indefinite number of query conditions, 1=1 can be very convenient specification statements. For example, a query may have name,age,height,weight constraints, or may not, how to deal with it?

String Sql=select * FROM table1 where 1=1

Why write an extra 1=1? I'll find out soon.

Copy Code code as follows:

if (!name.equals ("")) {
sql=sql+ "Name= '" "+name+" ";
}
if (!age.equals ("")) {
sql=sql+ "Age" "+age+" ";
}
if (!height.equals ("")) {
sql=sql+ "height= '" "+height+" ";
}
if (!weight.equals ("")) {
sql=sql+ "weight= '" "+weight+" ";
}

If you do not write 1=1, then in each of the query conditions that are not empty, must be judged there is no where sentence, or to be in the first place to add where

Where 1=1 is written in order to detect conditions in the process of checking
For example, there are three parameters a, B, c
@sql =select * from TB '
All three parameters may be empty
At this time you want to construct the statement, a test and then write a statement on the trouble
Like what
If @a is not null
@sql = @sql + "where a= ' + @a
If @b is not null
How do you write here? Do you want to add where or directly with and? You also need to detect if @a is empty

With the where 1=1, there is no such problem, provided that and is directly and, or is directly connected to or

Copy table
Create table_name AS SELECT * from source_table where 1=1;

Duplicate table structure
create   table_name   as   select   *    from   source_table   where   1 <> 1;