MySQL error "Error 1215: Cannot add foreign key constraint" is usually caused by the foreign key setting that does not meet the prerequisites. 1. Data type mismatch: The data type, length, and symbol attributes of the foreign key and the primary key field must be the same; 2. Engine inconsistency: Only the InnoDB engine supports foreign keys, so make sure that the table uses InnoDB; 3. Different character sets or sorting rules: It is recommended to unify the character set and sorting rules; 4. Fields are not indexed: Foreign key fields should be explicitly indexed; 5. Table or field names are spelled incorrectly: Check whether they exist and whether they are spelled correctly. During the investigation, the above reasons can be corrected one by one.
MySQL error "Error 1215: Cannot add foreign key constraint" is a common problem that many people encounter when creating tables or adding foreign keys. This problem is usually not a syntax error, but is caused by the foreign key settings that do not meet certain prerequisites.
Here are some common reasons and solutions, and directly add the information:
Data type mismatch
The foreign key field and the primary key field it references must have exactly the same data type and length , including whether it is signed/unsigned.
For example, if your primary key is INT UNSIGNED and your foreign key is INT , it will fail.
? Solution:
- Confirm whether the two fields are consistent
- Length, accuracy, and symbols must be matched one by one
Engine inconsistency (MyISAM and InnoDB)
Only the InnoDB engine in MySQL supports foreign key constraints . If your table is using MyISAM or other engine, an error will be reported even if the structure is not problem.
? Check method:
SHOW CREATE TABLE your_table_name;
? Modify the engine:
ALTER TABLE your_table_name ENGINE=InnoDB;
Character set or sorting rules are different (utf8mb4 vs utf8)
Although this effect is not as direct as the previous two, sometimes inconsistent character sets or collations can lead to failure in creating foreign keys, especially when cross-table references.
? It is recommended to unify character sets and collation rules, such as:
CREATE TABLE example (
id INT PRIMARY KEY
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;Fields have no index
If the foreign key field is not explicitly indexed, in some cases MySQL will automatically add one for you. But if the field has no index when you manually add a foreign key, an error may occur.
? It is recommended to add indexes in advance:
CREATE INDEX idx_name ON table_name(foreign_key_column);
Table does not exist or is spelled wrongly
Sometimes you just write the table or field name incorrectly, especially under case-sensitive systems, which are easily overlooked.
? Checkpoint:
- Is it spelled correctly
- Whether the table exists
- Is the field name wrong
Basically these common reasons. This type of problem is not complicated, but it is easy to get stuck due to negligence in details. When encountering 1215 errors, it is recommended to check them one by one according to the above points, and most cases can be solved.
The above is the detailed content of mysql error 1215 cannot add foreign key constraint. For more information, please follow other related articles on the PHP Chinese website!
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