create

The create statement only supports creating DFS databases.

create database directory partitioned by partitionType(partitionScheme),
[engine='OLAP'], [atomic='TRANS'], [chunkGranularity='TABLE']

Please refer to the related function database for details. The number of partitionType indicates the partition levels. You can specify one to three partitionType for a database and use more than one partitionType to create a composite database.

This statement only supports creating standard in-memory tables and DFS tables.

create table dbPath.tableName (
  schema[columnDescription]
)
[partitioned by partitionColumns],
[sortColumns],
[keepDuplicates=ALL],
[sortKeyMappingFunction],
[softDelete=false]

dbPath is a string indicating the path of a dfs database.

tableName is a string indicating the table name, or a vector indicating the table object.

schema indicates the table schema, including two columns: columnName and columnType.

columnDescription uses keywords to add a description to a column, including:

• comment adds a comment to a column;

• compress specifies the compression method, which includes:

  • "lz4"

  • "delta"

  • "zstd"

partitionColumns is a string or a string vector indicating the partitioning column(s). For a composite partition, partitionColumns is a string vector.

Parameters for TSDB storage engine only:

sortColumns is a required argument for TSDB engine. It is a string scalar/vector indicating the columns based on which the table is sorted.

sortKeyMappingFunction is a vector of unary functions. It has the same length as the number of sort keys. The specified mapping functions are applied to each sort key (i.e., the sort columns except for the temporal column) for dimensionality reduction.After the dimensionality reduction for the sort keys, records with a new sort key entry will be sorted based on the last column of sortColumns (the temporal column).

Note:

• It cannot be specified when creating a dimension table.

• Dimensionality reduction is performed when writing to disk, so specifying this parameter may affect write performance.

• The functions specified in sortKeyMappingFunction correspond to each and every sort key. If a sort key does not require dimensionality reduction, leave the corresponding element empty in the vector.

• If a mapping function is hashBucket AND the sort key to which it applies is a HASH partitioning column, make sure the number of Hash partitions is not divisible by hashBucket().buckets (or vice versa), otherwise the column values from the same HASH partition would be mapped to the same hash bucket after dimensionality reduction.

softDelete determines whether to enable soft delete for TSDB databases. The default value is false. To use it, keepDuplicates must be set to 'LAST'. It is recommended to enable soft delete for databases where the row count is large and delete operations are infrequent.

Please refer to the related function createPartitionedTable / createDimensionTable for details.

To create a temporary in-memory table, add keywords local temporary (case insensitive) to create:

create local temporary table  tableName(
schema
) [on commit preserve rows]

where,

tableName is a string indicating the table name, or a variable of the table object

schema is the table schema which contains 2 columns: columnName and columnType.

on commit preserve rows (optional) specifies that the temporary table is session-specific. It is case-insensitive.

Note:

• In DolphinDB, the create local temporary table statement is equivalent to create table as it creates a local temporary in-memory table that is only valid in the current session.

• Currently, global temporary tables and the keyword on commit delete rows are not supported.

(1) create an in-memory table

create table tb(
            id SYMBOL,
            val DOUBLE
    )
     go;   //Parse and run codes with the go statement first, otherwise an error of unrecognized variable tb will be reported.
     tb.schema()

    partitionColumnIndex->-1
    chunkPath->
    colDefs->
    name typeString typeInt comment
    ---- ---------- ------- -------
    id   SYMBOL     17
    val  DOUBLE     16

(2) create a DFS database in TSDB engine

if(existsDatabase("dfs://test")) dropDatabase("dfs://test")
create database "dfs://test" partitioned by VALUE(1..10), HASH([SYMBOL, 40]), engine='TSDB'

(3) create a DFS table

create table "dfs://test"."pt"(
        id INT,
        deviceId SYMBOL,
        date DATE[comment="time_col", compress="delta"],
        value DOUBLE,
        isFin BOOL
    )
    partitioned by ID, deviceID,
    sortColumns=[`deviceId, `date],
    keepDuplicates=ALL

     pt = loadTable("dfs://test","pt")
     pt.schema()
    keepDuplicates->ALL
    engineType->TSDB
    partitionColumnIndex->[0,1]
    ...

(4) create a dimension table

create table "dfs://test"."pt1"(
        id INT,
        deviceId SYMBOL,
        date DATE[comment="time_col", compress="delta"],
        value DOUBLE,
        isFin BOOL
    )
    sortColumns=[`deviceId, `date]

     pt1 = loadTable("dfs://test","pt1")
     pt1.schema()
    sortColumns->[deviceId,date]
    engineType->TSDB
    keepDuplicates->ALL
    chunkGranularity->TABLE
    partitionColumnIndex->-1
    ...

(5) Create a temporary in-memory table

create local temporary table "tb" (
        id SYMBOL,
        val DOUBLE
    ) on commit preserve rows
     tb.schema()

    partitionColumnIndex->-1
    chunkPath->
    colDefs->
    name typeString typeInt extra comment
    ---- ---------- ------- ----- -------
    id   SYMBOL     17
    val  DOUBLE     16