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Update docs for expression defaults #19685

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Update docs for expression defaults #19685

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97605c0
Update docs for expression defaults
dveeden Dec 13, 2024
2899343
Link to UUID best practices
dveeden Dec 13, 2024
96e87ec
Fix linter issue
dveeden Dec 13, 2024
618d264
Fix linter issue
dveeden Dec 13, 2024
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45 changes: 42 additions & 3 deletions data-type-default-values.md
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Expand Up @@ -36,7 +36,7 @@ Implicit defaults are defined as follows:

Starting from 8.0.13, MySQL supports specifying expressions as default values in the `DEFAULT` clause. For more information, see [Explicit default handling as of MySQL 8.0.13](https://dev.mysql.com/doc/refman/8.0/en/data-type-defaults.html#data-type-defaults-explicit).

Starting from v8.0.0, TiDB additionally supports specifying the following expressions as default values in the `DEFAULT` clause.
TiDB supports specifying the following expressions as default values in the `DEFAULT` clause.

* `UPPER(SUBSTRING_INDEX(USER(), '@', 1))`
* `REPLACE(UPPER(UUID()), '-', '')`
Expand All @@ -46,9 +46,48 @@ Starting from v8.0.0, TiDB additionally supports specifying the following expres
* `DATE_FORMAT(NOW(), '%Y-%m-%d %H.%i.%s')`
* `DATE_FORMAT(NOW(), '%Y-%m-%d %H:%i:%s')`
* `STR_TO_DATE('1980-01-01', '%Y-%m-%d')`
* [`CURRENT_TIMESTAMP()`](/functions-and-operators/date-and-time-functions.md), [`CURRENT_DATE()`](/functions-and-operators/date-and-time-functions.md): both with default fsp
* [`JSON_OBJECT()`](/functions-and-operators/json-functions.md), [`JSON_ARRAY()`](/functions-and-operators/json-functions.md), [`JSON_QUOTE()`](/functions-and-operators/json-functions.md)
* [`NEXTVAL()`](/functions-and-operators/sequence-functions.md#nextval)
* [`RAND()`](/functions-and-operators/numeric-functions-and-operators.md)
* [`UUID()`](/functions-and-operators/miscellaneous-functions.md#uuid), [`UUID_TO_BIN()`](/functions-and-operators/miscellaneous-functions.md#uuid_to_bin)
* [`VEC_FROM_TEXT()`](/vector-search-functions-and-operators.md#vec_from_text)

Starting from v8.0.0, TiDB additionally supports assigning default values to `BLOB`, `TEXT`, and `JSON` data types. However, you can only use expressions to set the default values for these data types. The following is an example of `BLOB`:
TiDB supports assigning default values to `BLOB`, `TEXT`, and `JSON` data types. However, you can only use expressions to set the default values for these data types and not literals. The following is an example of `BLOB`:

```sql
CREATE TABLE t2 (b BLOB DEFAULT (RAND()));
CREATE TABLE t2 (
b BLOB DEFAULT (RAND())
);
```

An example for using a UUID:

```sql
CREATE TABLE t3 (
uuid BINARY(16) DEFAULT (UUID_TO_BIN(UUID())),
name VARCHAR(255)
);
```

For more details on how to use UUID, see [UUID best practices](/best-practices/uuid.md).

An example for using JSON:

```sql
CREATE TABLE t4 (
id bigint AUTO_RANDOM PRIMARY KEY,
j json DEFAULT (JSON_OBJECT("a", 1, "b", 2))
);
```

An example for what is not allowed for JSON:

```sql
CREATE TABLE t5 (
id bigint AUTO_RANDOM PRIMARY KEY,
j json DEFAULT ('{"a": 1, "b": 2}')
);
```

The last two examples describe a similar default, but only the first one is allowed as it is using an expression and not a literal.
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