Is IN operator equivalent to Equal operator in SOQL when binding a list variable?

Question

While testing some SOQL queries I noticed that the Equal operator behaves like the IN operator when using a list bind variable

All the queries below run without any error and return the same result

List<String> accNames=new List<String>{'Acme Corporation','Smith Enterprises'};
System.debug('Dynamic Query bind list with Equal operator :'+Database.query('select Name from account where Name = :accNames'));
System.debug('Static Query bind list with Equal operator :'+[select name from account where name = :accNames]);
System.debug('Dynamic Query bind list with IN operator :'+Database.query('select Name from account where Name in :accNames'));
System.debug('Static Query bind list with IN operator :'+[select name from account where name in :accNames]);

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Debug

DEBUG|Dynamic Query bind list with Equal operator :(Account:{Name=Acme Corporation, Id=0010Y00000E7whoQAB}, Account:{Name=Smith Enterprises, Id=0010Y00000E7wXsQAJ})
DEBUG|Static Query bind list with Equal operator :(Account:{Name=Acme Corporation, Id=0010Y00000E7whoQAB}, Account:{Name=Smith Enterprises, Id=0010Y00000E7wXsQAJ})
DEBUG|Dynamic Query bind list with IN operator :(Account:{Name=Acme Corporation, Id=0010Y00000E7whoQAB}, Account:{Name=Smith Enterprises, Id=0010Y00000E7wXsQAJ})
DEBUG|Static Query bind list with IN operator :(Account:{Name=Acme Corporation, Id=0010Y00000E7whoQAB}, Account:{Name=Smith Enterprises, Id=0010Y00000E7wXsQAJ})

Are the IN and = operators equal when using a list bind variable?

Answer 1

Yes, = and IN operators are functionally equivalent when filtering against a list. It is not well covered in the Force.com SOQL and SOSL Reference for Comparison Operators, but the operators are interchangeable as far as I have observed, if the bind variable is a List.

One notable exception is when using a Left Inner Join, where only IN will work.

Valid

SELECT Id FROM Profile WHERE Id IN (SELECT ProfileId FROM User)

Invalid

SELECT Id FROM Profile WHERE Id = (SELECT ProfileId FROM User)

See also: How to find out if an Object is a collection or single value?