CREATE TABLE
CREATE TABLE is the most complicated part of many Databases, you need to:
- Manually specify the engine
- Manually specify the indexes
- And even specify the data partitions or data shard
In Databend, you don't need to specify any of these, one of Databend's design goals is to make it easier to use.
Syntax
Create Table
CREATE [TRANSIENT] TABLE [IF NOT EXISTS] [db.]table_name
(
<column_name> <data_type> [ NOT NULL | NULL] [ { DEFAULT <expr> }],
<column_name> <data_type> [ NOT NULL | NULL] [ { DEFAULT <expr> }],
...
) [CLUSTER BY(<expr> [, <expr>, ...] )]
<data_type>:
TINYINT
| SMALLINT
| INT
| BIGINT
| FLOAT
| DOUBLE
| DATE
| TIMESTAMP
| VARCHAR
| ARRAY
| OBJECT
| VARIANT
tip
For detailed information about the CLUSTER BY clause, see SET CLUSTER KEY.
CREATE TABLE ... LIKE
Creates an empty copy of an existing table, the new table automatically copies all column names, their data types, and their not-null constraints.
Syntax:
CREATE TABLE [IF NOT EXISTS] [db.]table_name
LIKE [db.]origin_table_name
CREATE TABLE ... AS [SELECT query]
Creates a table and fills it with data computed by a SELECT command.
Syntax:
CREATE TABLE [IF NOT EXISTS] [db.]table_name
LIKE [db.]origin_table_name
AS SELECT query
CREATE TRANSIENT TABLE ...
Creates a transient table.
Transient tables are used to hold transitory data that does not require a data protection or recovery mechanism. Dataebend does not hold historical data for a transient table so you will not be able to query from a previous version of the transient table with the Time Travel feature, for example, the AT clause in the SELECT statement will not work for transient tables. Please note that you can still drop and undrop a transient table.
Transient tables help save your storage expenses because they do not need extra space for historical data compared to non-transient tables. See example for detailed explanations.
Syntax:
CREATE TRANSIENT TABLE ...
Column Nullable
By default, all columns are not nullable(NOT NULL), if you want to specify a column default to NULL, please use:
CREATE TABLE [IF NOT EXISTS] [db.]table_name
(
<column_name> <data_type> NULL,
...
)
Let check it out how difference the column is NULL or NOT NULL.
Create a table t_not_null which column with NOT NULL(Databend Column is NOT NULL by default):
CREATE TABLE t_not_null(a INT);
DESC t_not_null;
+-------+-------+------+---------+
| Field | Type | Null | Default |
+-------+-------+------+---------+
| a | Int32 | NO | 0 |
+-------+-------+------+---------+
Create another table t_null column with NULL:
CREATE TABLE t_null(a INT NULL);
DESC t_null;
+-------+-------+------+---------+
| Field | Type | Null | Default |
+-------+-------+------+---------+
| a | Int32 | YES | NULL |
+-------+-------+------+---------+
Default Values
DEFAULT <expression>
Specifies a default value inserted in the column if a value is not specified via an INSERT or CREATE TABLE AS SELECT statement.
For example:
CREATE TABLE t_default_value(a TINYINT UNSIGNED, b SMALLINT DEFAULT (a+3), c VARCHAR DEFAULT 'c');
Desc the t_default_value table:
DESC t_default_value;
+-------+--------+------+---------+
| Field | Type | Null | Default |
+-------+--------+------+---------+
| a | UInt8 | NO | 0 |
| b | Int16 | NO | (a + 3) |
| c | String | NO | c |
+-------+--------+------+---------+
Insert a value:
INSERT INTO T_default_value(a) VALUES(1);
Check the table values:
SELECT * FROM t_default_value;
+------+------+------+
| a | b | c |
+------+------+------+
| 1 | 4 | c |
+------+------+------+
MySQL Compatibility
Databend’s syntax is difference from MySQL mainly in the data type and some specific index hints.
Examples
Create Table
CREATE TABLE test(a BIGINT UNSIGNED, b VARCHAR , c VARCHAR DEFAULT concat(b, '-b'));
DESC test;
+-------+--------+------+---------------+
| Field | Type | Null | Default |
+-------+--------+------+---------------+
| a | UInt64 | NO | 0 |
| b | String | NO | |
| c | String | NO | concat(b, -b) |
+-------+--------+------+---------------+
INSERT INTO test(a,b) VALUES(888, 'stars');
SELECT * FROM test;
+------+-------+---------+
| a | b | c |
+------+-------+---------+
| 888 | stars | stars-b |
+------+-------+---------+
Create Table Like Statement
CREATE TABLE test2 LIKE test;
DESC test2;
+-------+--------+------+---------------+
| Field | Type | Null | Default |
+-------+--------+------+---------------+
| a | UInt64 | NO | 0 |
| b | String | NO | |
| c | String | NO | concat(b, -b) |
+-------+--------+------+---------------+
INSERT INTO test2(a,b) VALUES(888, 'stars');
SELECT * FROM test2;
+------+-------+---------+
| a | b | c |
+------+-------+---------+
| 888 | stars | stars-b |
+------+-------+---------+
Create Table As SELECT (CTAS) Statement
CREATE TABLE test3 AS SELECT * FROM test2;
DESC test3;
+-------+--------+------+---------------+
| Field | Type | Null | Default |
+-------+--------+------+---------------+
| a | UInt64 | NO | 0 |
| b | String | NO | |
| c | String | NO | concat(b, -b) |
+-------+--------+------+---------------+
SELECT * FROM test3;
+------+-------+---------+
| a | b | c |
+------+-------+---------+
| 888 | stars | stars-b |
+------+-------+---------+
Create Transient Table
-- Create a transient table
CREATE TRANSIENT TABLE mytemp (c bigint);
-- Insert values
insert into mytemp values(1);
insert into mytemp values(2);
insert into mytemp values(3);
-- Only one snapshot is stored. This explains why the Time Travel feature does not work for transient tables.
select count(*) from fuse_snapshot('default', 'mytemp');
+---------+
| count() |
+---------+
| 1 |