MySQL throws error 3156 when a JSON value cannot be converted to the target type using CAST or CONVERT.
MySQL error 3156 ER_INVALID_JSON_VALUE_FOR_CAST appears when CAST or CONVERT encounters JSON that does not fit the requested data type. Validate the JSON path, ensure the value matches the target type, or pre-sanitize it before casting to resolve the issue.
ER_INVALID_JSON_VALUE_FOR_CAST
Error 3156 appears when MySQL attempts to CAST or CONVERT a JSON value into another data type and the chosen value cannot be interpreted as that type. The server aborts the statement and returns SQLSTATE 22018.
The problem was introduced in MySQL 5.7.8 alongside native JSON support. It often surfaces in UPDATE queries, computed columns, or SELECT statements that include JSON_EXTRACT and CAST together.
The error fires during any statement that extracts a JSON scalar and immediately casts it to a numeric, date, or character type. If the extracted fragment is null, an object, an array, or a malformed literal, MySQL cannot perform the conversion and raises error 3156.
It can also occur in generated columns defined with CAST(JSON_EXTRACT(...)) if incoming rows contain incompatible JSON values.
Left unfixed, the error blocks data ingestion, ETL jobs, and application features that rely on JSON data. It may hide data quality issues or signal an incorrect schema design. Quick resolution keeps pipelines flowing and ensures data integrity.
Casting an object or array rather than a scalar (string, number, boolean) triggers the error because complex types cannot map directly to numeric or date types.
Values like "abc" or "12,34" inside JSON produce an invalid cast when converted to DECIMAL or INT.
Strings such as "2024/31/02" or "99:99:99" do not match MySQL date or time formats and cause the cast to fail.
When JSON_EXTRACT returns NULL and the target column is NOT NULL, the cast fails during inserts or updates.
Raised when the entire JSON string is invalid, unlike error 3156 which concerns casting.
Occurs when the JSON path expression is malformed or references an illegal character.
Appears when the JSON document contains illegal characters outside the casting context.
Yes. Although added in 5.7.8, the error remains in 8.0 when you cast incompatible JSON values.
You can wrap CAST inside NULLIF or use IFNULL(JSON_UNQUOTE(...),'0') before casting, but be sure this logic fits your business rules.
Galaxy's AI copilot inspects JSON paths, previews extracted values, and flags non-scalar fragments before you run the query, reducing runtime errors.
Minimal for small datasets, but heavy use of JSON_TYPE and JSON_VALID can slow large scans. Index generated columns to offset the cost.
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