[firebird-support] How to insert only if a matching row does not exist?

[Ed Grochowski]

Using  SQL only (no stored procedures), how do I construct a statement to 
insert values into a database table, if and only if, a row containing a field 
with a specific value does not already exist?

In Sql Server (and others), you can do things like the following:

IFNOTEXISTS(SELECT1FROMemp WHEREfruits ='mango')
    INSERTINTOemp (fruits)VALUES('mango')

What is the analogous way to achieve this in Firebird (2.5) ?

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