By default, MySQL is forgiving of invalid or improper data values and coerces them to valid values for data entry. However, you can enable strict SQL mode to select more traditional treatment of bad values such that the server rejects them and aborts the statement in which they occur. See Section 5.1.11, “Server SQL Modes”.
This section describes the default (forgiving) behavior of MySQL, as well as the strict SQL mode and how it differs.
If you are not using strict mode, then whenever you insert an
“incorrect” value into a column, such as a
NULL into a
column or a too-large numeric value into a numeric column,
MySQL sets the column to the “best possible
value” instead of producing an error: The following
rules describe in more detail how this works:
If you try to store an out of range value into a numeric column, MySQL Server instead stores zero, the smallest possible value, or the largest possible value, whichever is closest to the invalid value.
For strings, MySQL stores either the empty string or as much of the string as can be stored in the column.
If you try to store a string that does not start with a number into a numeric column, MySQL Server stores 0.
MySQL permits you to store certain incorrect date values into
DATETIMEcolumns (such as
'2000-02-00'). In this case, when an application has not enabled strict SQL mode, it up to the application to validate the dates before storing them. If MySQL can store a date value and retrieve exactly the same value, MySQL stores it as given. If the date is totally wrong (outside the server's ability to store it), the special “zero” date value
'0000-00-00'is stored in the column instead.
If you try to store
NULLinto a column that doesn't take
NULLvalues, an error occurs for single-row
INSERTstatements. For multiple-row
INSERTstatements or for
INSERT INTO ... SELECTstatements, MySQL Server stores the implicit default value for the column data type. In general, this is
0for numeric types, the empty string (
'') for string types, and the “zero” value for date and time types. Implicit default values are discussed in Section 11.7, “Data Type Default Values”.
INSERTstatement specifies no value for a column, MySQL inserts its default value if the column definition includes an explicit
DEFAULTclause. If the definition has no such
DEFAULTclause, MySQL inserts the implicit default value for the column data type.
The reason for using the preceding rules in nonstrict mode is that we can't check these conditions until the statement has begun executing. We can't just roll back if we encounter a problem after updating a few rows, because the storage engine may not support rollback. The option of terminating the statement is not that good; in this case, the update would be “half done,” which is probably the worst possible scenario. In this case, it is better to “do the best you can” and then continue as if nothing happened.
SET sql_mode = 'STRICT_TRANS_TABLES'; SET sql_mode = 'STRICT_ALL_TABLES';
strict mode for transactional storage engines, and also to
some extent for nontransactional engines. It works like this:
For transactional storage engines, bad data values occurring anywhere in a statement cause the statement to abort and roll back.
For nontransactional storage engines, a statement aborts if the error occurs in the first row to be inserted or updated. (When the error occurs in the first row, the statement can be aborted to leave the table unchanged, just as for a transactional table.) Errors in rows after the first do not abort the statement, because the table has already been changed by the first row. Instead, bad data values are adjusted and result in warnings rather than errors. In other words, with
STRICT_TRANS_TABLES, a wrong value causes MySQL to roll back all updates done so far, if that can be done without changing the table. But once the table has been changed, further errors result in adjustments and warnings.
For even stricter checking, enable
STRICT_ALL_TABLES. This is
the same as
that for nontransactional storage engines, errors abort the
statement even for bad data in rows following the first row.
This means that if an error occurs partway through a
multiple-row insert or update for a nontransactional table, a
partial update results. Earlier rows are inserted or updated,
but those from the point of the error on are not. To avoid
this for nontransactional tables, either use single-row
statements or else use
conversion warnings rather than errors are acceptable. To
avoid problems in the first place, do not use MySQL to check
column content. It is safest (and often faster) to let the
application ensure that it passes only valid values to the