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Understanding and Resolving "Invalid default value for timestamp" in MySQL 5.7

The "Invalid default value for timestamp" error in MySQL 5.7 and earlier is a common stumbling block for developers. It usually surfaces when you're trying to create a table with a `TIMESTAMP` column and specifying a default value that MySQL considers invalid. This tutorial will dissect the error, explain its causes, and provide various solutions with code examples.

*1. The Root of the Problem: `explicit_defaults_for_timestamp`*

The behavior of `TIMESTAMP` columns in MySQL 5.7 hinges on a server SQL mode setting called `explicit_defaults_for_timestamp`. This setting significantly alters how MySQL handles default and `ON UPDATE CURRENT_TIMESTAMP` behaviors for `TIMESTAMP` columns.

*Without `explicit_defaults_for_timestamp` (Default in MySQL 5.6 and earlier, and often the cause of the problem in upgrading to 5.7):*

If you define the first `TIMESTAMP` column in a table without explicitly specifying `NOT NULL` and `DEFAULT` (or `ON UPDATE CURRENT_TIMESTAMP`), MySQL implicitly makes it `NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP`. In essence, MySQL takes over the management of that column, making it automatically record the timestamp of the row's creation and update.
If you try to define a different column as `TIMESTAMP` with `DEFAULT CURRENT_TIMESTAMP` (or `ON UPDATE CURRENT_TIMESTAMP`), it may be flagged as an error since the first `TIMESTAMP` column is already fulfilling the automatic update behavior.
You can only have one `TIMESTAMP` column with the `ON UPDATE CURRENT_TIMESTAMP` attribute.

*With `explicit_defaults_for_timestamp` (Recommended and default in MySQL 8.0 and later):*

`TIMESTAMP` columns behave like any other data type. You must explicitly define `NOT NULL` and `DEFAULT` (or `ON UPDATE CURRENT_TIMESTAMP`) if you want those behaviors. MySQL will not automatically assign default values or `ON UPDATE` clauses.
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