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Editorial Team
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Asked: 2026-06-13T02:14:47+00:00

I m trying to add a foreign key to table table2 from table1 .

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I m trying to add a foreign key to table table2 from table1. But i got an error like this… what is the problem…help

Error

SQL query:

ALTER TABLE `table2` ADD FOREIGN KEY ( `bus_id` ) REFERENCES `db1`.`table1` (
`bus_id`
) ON DELETE CASCADE ON UPDATE CASCADE ;

MySQL said: Documentation
#1452 - Cannot add or update a child row: a foreign key constraint fails (`db1`.`#sql-664_e2`, CONSTRAINT `#sql-664_e2_ibfk_1` FOREIGN KEY (`bus_id`) REFERENCES `table1` (`bus_id`) ON DELETE CASCADE ON UPDATE CASCADE)

Structure of table1

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Structure of table2

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1 Answer

  1. Editorial Team

    Your tables have some data. Check that all values stored in table2.bus_id are correct; all these values have to be in table1.bus_id.

    Run this query to view wrong bus_id values in table2:

    SELECT t2.bus_id AS inconsistent_bus_id FROM table2 t2
LEFT JOIN table1 t1
  ON t1.bus_id = t2.bus_id
WHERE t1.bus_id IS NULL

    Remove this records from table2, or add this bus_id to the table1. Then try to create foreign key again.

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