SQL syntax appears correct but EXPLAIN does not work?

[David Wynter]

This query here will run (in fact for hours) so I wanted to find out why it is so slow.

UPDATE SECM_SECURITY_UNIVERSE, PREFERRED_STOCK_INFORMATION, BOND_CORE_INFORMATION, CONVERTIBLE_BOND_INFO
SET SECM_SECURITY_UNIVERSE.STAMP = NOW()
WHERE (SECM_SECURITY_UNIVERSE.ASSET_ID = PREFERRED_STOCK_INFORMATION.ASSET_ID AND PREFERRED_STOCK_INFORMATION.PROCESSED>0)
OR (SECM_SECURITY_UNIVERSE.ASSET_ID = BOND_CORE_INFORMATION.ASSET_ID AND BOND_CORE_INFORMATION.PROCESSED>0)
OR (SECM_SECURITY_UNIVERSE.ASSET_ID = CONVERTIBLE_BOND_INFO.ASSET_ID AND CONVERTIBLE_BOND_INFO.PROCESSED>0)

I tried it in MySQL Query Browser and it gives a syntax error message from the start of the statement. I tried it in the cmd line MySQL window with a EXPLAIN on the front to, same error? So how come it will run but EXPLAIN will not work on it?

Thx,

David