如何在 MySQL 'insert' 语句中使用 'select'

Question

I'm trying to insert additional rows into a table which requires a value to be retrieved from another table. Below is an example query:

insert into a.grades (rollno, grade)
values(select rollno from b.students where ssn=12345, 'A');

Structure of b.students table is rollno, ssn, name .

I knew the above query is wrong. Is there a way to retrieve 1 value from other table while inserting a row?

Answer 1

INSERT INTO a.grades (rollno, grade)
    SELECT rollno, 'A' FROM b.students WHERE ssn = 12345;

Some DBMS would accept the following, with an extra set of parenthesis around the SELECT statement:

INSERT INTO a.grades (rollno, grade)
   VALUES((SELECT rollno FROM b.students WHERE ssn = 12345), 'A');

Answer 2

Columns in insert into and select must be equal

INSERT INTO grades (field1, field2)
  SELECT field1, field2 from students where ssn=12345;

Answer 3

Tables from two different databases!

Database1 - person

Database2 - order

• Table - per_details
• Table - or_details

Here the insert query used under Database 2!

INSERT INTO `or_details`(`per_name`) VALUES ( (SELECT person.per_details.per_name from person.per_details WHERE person.per_details.id=1001) );