Why is the query not executed when populating the SQL table?

Question

SQL

Given a table with an index:
create table TABLE_1 (col1 int, col2 int);
create unique index INDEX_1_2 on TABLE_1(col1, col2);
We start filling it with data by executing the queries below one by one. Which of the queries will succeed? Why won't any query be executed?
insert into TABLE_1 values ​​(1, 2);
insert into TABLE_1 values ​​(null, 1);
insert into TABLE_1 values ​​(null, null);
insert into TABLE_1 values ​​(null, 1);

Answer 1

all queries
will be executed You will not execute only the query in which there will be any values ​​other than NULL in both fields and already existing in the database. None of the conditions match
. Executing it insert into TABLE_1 values (1, 2);again will result in an error.