What is the difference between keeping column on left of = in sql

Question

I am reading someone else sql and his code was like this

There is view called user_v with column path as Array

select * from user_v where 'USER_TYPE'=path[2]

can't i use

path[2] = 'USER_TYPE'

Answer 1

This is a precaution taken by some programmers in languages where assignment and comparison can be easily confused, such as C or PHP, where the following statement looks innocent:

 if ( $foo = 1 )

but it is actually assigning 1 to $foo , and the if will always evaluate to true (in PHP, at least, where 1 is true). What was meant was if ( $foo == 1 ) .

If you reverse the arguments, the error becomes obvious sooner:

if ( 1 = $foo ) # SYNTAX ERROR
if ( 1 == $foo ) # Desired behaviour

This is sometimes known as "yoda coding", and is in the coding standards of Wordpress, for example .

See also: Why put the constant before the variable in a comparison?

In SQL, there is less chance of such a muddle, since although = can mean either assignment or comparison, there are rarely situations where a typo would select the wrong meaning.

However, if the coding standards for every other language used by a project mandate it, it would make sense to reinforce the habit by also using it in SQL, since I can't think of a specific reason not to write it that way.

Answer 2

完全没有区别。

Answer 3

It's psychology. You would want to read someone else's code out laud and say:

Where my column equals 2.

When you read:

Where 2 equals my column

you have to stop for a while, return, explain it to yourself.

We maintain all of these rules that seem rubish at first glance just to make other people lives easier.