Condtional insert using a 'WHERE' from another table - not the one inserted into

I want to be able to insert into one table but only if a condition is met on another table.

for example something like this

INSERT INTO table_1 (table_2_id,y,z) VALUES (123,2,3) 

WHERE (
(SELECT COUNT(table_2_id) FROM table_2 WHERE valid=1 and id=123)=1
)

Basically - I want to only insert into table_1 if the related record 开发者_Python百科in table_2 is valid else it should fail.

is there a way to do this in one query or will I have to use some php+seperate query to do the check instead?

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See whether this works for you:

INSERT INTO table_1 (table_2_id,y,z) 
SELECT '123','2','3' 
FROM table_2 WHERE valid=1 and id=123