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Merge pull request #46836 from den-crane/patch-64

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Doc. Examples for DEFAULT/MATERIALIZED/EPHEMERAL/ALIAS
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Alexey Milovidov 2023-02-26 00:26:11 +03:00 committed by GitHub
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docs/en/sql-reference/statements/create/table.md
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@ -127,6 +127,26 @@ Default expressions may be defined as an arbitrary expression from table constan
Normal default value. If the INSERT query does not specify the corresponding column, it will be filled in by computing the corresponding expression.
Example:
```sql
CREATE OR REPLACE TABLE test
(
id UInt64,
updated_at DateTime DEFAULT now(),
updated_at_date Date DEFAULT toDate(updated_at)
)
ENGINE = MergeTree
ORDER BY id;
INSERT INTO test (id) Values (1);
SELECT * FROM test;
┌─id─┬──────────updated_at─┬─updated_at_date─┐
│ 1 │ 2023-02-24 17:06:46 │ 2023-02-24 │
└────┴─────────────────────┴─────────────────┘
```
### MATERIALIZED
`MATERIALIZED expr`
@ -135,6 +155,36 @@ Materialized expression. Such a column can’t be specified for INSERT, because
For an INSERT without a list of columns, these columns are not considered.
In addition, this column is not substituted when using an asterisk in a SELECT query. This is to preserve the invariant that the dump obtained using `SELECT *` can be inserted back into the table using INSERT without specifying the list of columns.
Example:
```sql
CREATE OR REPLACE TABLE test
(
id UInt64,
updated_at DateTime MATERIALIZED now(),
updated_at_date Date MATERIALIZED toDate(updated_at)
)
ENGINE = MergeTree
ORDER BY id;
INSERT INTO test Values (1);
SELECT * FROM test;
┌─id─┐
│ 1 │
└────┘
SELECT id, updated_at, updated_at_date FROM test;
┌─id─┬──────────updated_at─┬─updated_at_date─┐
│ 1 │ 2023-02-24 17:08:08 │ 2023-02-24 │
└────┴─────────────────────┴─────────────────┘
SELECT * FROM test SETTINGS asterisk_include_materialized_columns=1;
┌─id─┬──────────updated_at─┬─updated_at_date─┐
│ 1 │ 2023-02-24 17:08:08 │ 2023-02-24 │
└────┴─────────────────────┴─────────────────┘
```
### EPHEMERAL
`EPHEMERAL [expr]`
@ -142,6 +192,34 @@ In addition, this column is not substituted when using an asterisk in a SELECT q
Ephemeral column. Such a column isn't stored in the table and cannot be SELECTed, but can be referenced in the defaults of CREATE statement. If `expr` is omitted type for column is required.
INSERT without list of columns will skip such column, so SELECT/INSERT invariant is preserved - the dump obtained using `SELECT *` can be inserted back into the table using INSERT without specifying the list of columns.
Example:
```sql
CREATE OR REPLACE TABLE test
(
id UInt64,
unhexed String EPHEMERAL,
hexed FixedString(4) DEFAULT unhex(unhexed)
)
ENGINE = MergeTree
ORDER BY id
INSERT INTO test (id, unhexed) Values (1, '5a90b714');
SELECT
id,
hexed,
hex(hexed)
FROM test
FORMAT Vertical;
Row 1:
──────
id: 1
hexed: Z<><5A>
hex(hexed): 5A90B714
```
### ALIAS
`ALIAS expr`
@ -156,6 +234,29 @@ If you add a new column to a table but later change its default expression, the
It is not possible to set default values for elements in nested data structures.
```sql
CREATE OR REPLACE TABLE test
(
id UInt64,
size_bytes Int64,
size String Alias formatReadableSize(size_bytes)
)
ENGINE = MergeTree
ORDER BY id;
INSERT INTO test Values (1, 4678899);
SELECT id, size_bytes, size FROM test;
┌─id─┬─size_bytes─┬─size─────┐
│ 1 │ 4678899 │ 4.46 MiB │
└────┴────────────┴──────────┘
SELECT * FROM test SETTINGS asterisk_include_alias_columns=1;
┌─id─┬─size_bytes─┬─size─────┐
│ 1 │ 4678899 │ 4.46 MiB │
└────┴────────────┴──────────┘
```
## Primary Key
You can define a [primary key](../../../engines/table-engines/mergetree-family/mergetree.md#primary-keys-and-indexes-in-queries) when creating a table. Primary key can be specified in two ways: