MySQL Error 1215: Cannot add foreign key constraint between Parent and Children - mysql

Question

I am utterly disappointed as I failed to pinpoint to the exact reason, why my table creation is failing when I am trying to create a Foreign Key relation between my two tables. The SQL queries that I am using to create the 2 tables are as under:
CREATE TABLE `OneMD_DEA_EMEA_STG_CUSTOMER` (
`Customer_Pkey` int(11) NOT NULL AUTO_INCREMENT,
`CustomerID` varchar(15) NOT NULL,
`DataType` varchar(10) NOT NULL,
`SourceSystemName` varchar(10) NOT NULL,
`SourceCountry` varchar(2) NOT NULL,
`SrcDataRfrshDt` date NOT NULL,
`StartDt` date NOT NULL,
`EndDt` date NOT NULL,
`Category` varchar(50) DEFAULT NULL,
PRIMARY KEY (`Customer_Pkey`,`CustomerID`),
UNIQUE KEY `CustomerID_UNIQUE` (`CustomerID`),
KEY `idx_OneMD_DEA_EMEA_STG_CUSTOMER` (`DataType`,`SrcDataRfrshDt`,`SourceCountry`,`EndDt`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
CREATE TABLE `STG_RELATION` (
`Relation_Pkey` int(11) NOT NULL AUTO_INCREMENT,
`RelationType` varchar(15) NOT NULL,
`SourceDataType` varchar(5) NOT NULL,
`SourceID` varchar(15) NOT NULL,
`TargetDataType` varchar(5) NOT NULL,
`TargetID` varchar(15) NOT NULL,
`RltnPrmryID` varchar(50) NOT NULL,
`SourceSystemName` varchar(10) DEFAULT NULL,
`SourceCountry` varchar(2) NOT NULL,
`SrcDataRfrshDt` date NOT NULL,
`StartDt` date NOT NULL,
`EndDt` date NOT NULL,
`HCPHCOSubType` varchar(10) DEFAULT NULL,
`HCPHCOLinkType` varchar(10) DEFAULT NULL,
`HCOHCOSubType` varchar(10) DEFAULT NULL,
`HCOHCOLinkType` varchar(10) DEFAULT NULL,
PRIMARY KEY (`Relation_Pkey`,`SourceID`,`TargetID`,`RltnPrmryID`),
UNIQUE KEY `Relation_Pkey_UNIQUE` (`Relation_Pkey`),
KEY `idx_STG_RELATION` (`RelationType`,`SrcDataRfrshDt`,`SourceCountry`,`EndDt`),
KEY `Source_ID_idx` (`SourceID`),
KEY `Target_ID_idx` (`TargetID`),
CONSTRAINT `Source_ID` FOREIGN KEY (`SourceID`) REFERENCES `STG_CUSTOMER` (`CustomerID`) ON DELETE CASCADE ON UPDATE NO ACTION,
CONSTRAINT `Target_ID` FOREIGN KEY (`TargetID`) REFERENCES `STG_CUSTOMER` (`CustomerID`) ON DELETE CASCADE ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
CREATE TABLE `STG_RELATION_ROLE` (
`RltnRole_PKey` int(11) NOT NULL AUTO_INCREMENT,
`SourceSystemName` varchar(10) NOT NULL,
`ActivityID` varchar(15) NOT NULL,
`RltnFrgnID` varchar(50) NOT NULL,
`SrcDataRfrshDt` date NOT NULL,
`StartDt` date NOT NULL,
`EndDt` date NOT NULL,
`SourceCountry` varchar(2) NOT NULL,
`HCPHCORoleType` varchar(10) DEFAULT NULL,
`HCPHCORoleField` varchar(10) DEFAULT NULL,
`HCPHCORole` varchar(10) DEFAULT NULL,
`HCPHCORoleStatus` varchar(20) DEFAULT NULL,
PRIMARY KEY (`RltnRole_PKey`,`SourceSystemName`,`ActivityID`,`RltnFrgnID`),
KEY `idx_STG_RELATION_ROLE` (`SrcDataRfrshDt`,`SourceSystemName`,`SourceCountry`,`EndDt`),
KEY `Rltn_Frgn_ID_idx` (`RltnFrgnID`),
CONSTRAINT `RltnFrgnID` FOREIGN KEY (`RltnFrgnID`) REFERENCES `STG_RELATION` (`RltnPrmryID`) ON DELETE CASCADE ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
The Table RELATION got created successfully, but whenver I am trying to create the child table (RELATION_ROLE with parent as RELATION), the table creation is failing with the
Error 1215: Cannot Add Foreign Key Constraint
error message.
Am I missing something here?
Please note that CUSTOMER is the main table with child as RELATION (Customer_ID acting as Primary and Foreign Key) which further has a child RELATION_ROLE (RltnPrmryID is the Primary Key while RltnFrgnID is the foreign key.
Please help me to get the issue resolved.

Answer 1

Uh, there are quite a few problems there - in particular with your table designs.
First of all, the STG_RELATION.RltnPrmryID is not the left most field of any indexes. MySQL requires both endpoints of a foreign key to be the leftmost fields of an index. The leftmost fields are the ones that can be used for lookups. This is the direct reason of the error.
Secondly, the primary key of STG_RELATION table is a mess. The child table should reference the primary key or a unique key from the parent table. STG_RELATION.RltnPrmryID is neither, it is only part of the primary key. Relation_Pkey field being auto increment already uniquely identifies records within the STG_RELATION table, therefore this should be the PK and STG_RELATION_ROLE table should reference this column in the foreign key (obviously, you need to adjust the field type for this to work in STG_RELATION_ROLE). Indexes on integers are a lot more efficient than indexes on strings from a performance point of view.