在将数据添加到 mysql 数据库中的表之前检查

Question

I am trying to insert data in table emp_master where token_no and vehicle_no is available, I am using mentioned below query to add and check the record

insert into emp_master (token_no, vehicle_no) values (1234,12345) where not exists (select * from emp_master where vehicle_no = '12345');

but whenever I am trying this then error comes in as

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'where not exists (select * from emp_master where vehicle_no = '12345')' at line 1

The suggestion will be helpful

Answer 1

If you need to have this check in your insert statement, you can structure it like this:

SQL Fiddle : http://sqlfiddle.com/#!9/d3dd77/1

insert into emp_master (token_no, vehicle_no) 
select 1234, 12345
from dual
where not exists (select * from emp_master where vehicle_no = '12345');

Another way to do this on any DBMS:

DB Fiddle Demo

insert into emp_master (token_no, vehicle_no) 
select token_no, vehicle_no
from (
select 1234 token_no, 12345 vehicle_no
) rec
where not exists (select * from emp_master where vehicle_no = rec.vehicle_no);

Answer 2

Put a UNIQUE constraint on the vehicle_no column, and then insert the data with no WHERE clause. Instead, handle the exception if it fails.

Answer 3

You can use an INSERT ... SELECT query to achieve this functionality:

INSERT INTO emp_master (token_no, vehicle_no) 
SELECT 1234, 12345
WHERE NOT EXISTS (SELECT * FROM emp_master WHERE vehicle_no = '12345')

Demo on dbfiddle