SQL to Blaze — Anaconda Enterprise 4.4 documentation

Anaconda Enterprise: Documentation

# SQL to Blaze

This page maps SQL expressions to blaze expressions.

Note

The following SQL expressions are somewhat specific to PostgreSQL, but blaze itself works with any database for which a SQLAlchemy dialect exists.

## Prerequisites

If you’re interested in testing these against a PostgreSQL database, make sure you’ve executed the following code in `psql` session:

    CREATE TABLE df (
        id BIGINT,
        amount DOUBLE PRECISION,
        name TEXT
    );

On the blaze side of things, the table below assumes the following code has been executed:

    >>> from blaze import symbol, by, join, concat
    >>> df = symbol('df', 'var * {id: int64, amount: float64, name: string}')

Note

Certain SQL constructs such as window functions don’t directly correspond to a particular Blaze expression. `Map` expressions are the closest representation of window functions in Blaze.

Computation | SQL | Blaze
--- | --- | ---
Column Arithmetic |
    select amount * 2 from df
    df.amount * 2
Multiple Columns |
    select id, amount from df
    df[['id', 'amount']]
Selection |
    selelct * from df where amount > 300
    df[df.amount > 300]
Group By |
    select avg(amount) from df group by name
    by(df.name, amount=df.amount.mean())
    select avg(amount) from df group by name, id
    by(merge(df.name, df.id),
       amount=df.amount.mean())
Join |
    select * from
        df inner join df2
    on df.name = df2.name
    join(df, df2, 'name')
Map |
    select amount + 1 over () from df
    df.amount.map(lambda x: x + 1,
                  'int64')
Relabel Columns |
    select
        id,
        name as alias,
        amount as dollars
     from df
    df.relabel(name='alias',
               amount='dollars')
Drop duplicates |
    select distinct * from df
    df.distinct()
    select distinct(name) from df
    df.name.distinct()
    /* postgresql only */
    select distinct on (name) * from
    df order by name
Reductions |
    select avg(amount) from df
    df.amount.mean()
    select amount, count(amount)
    from df group by amount
    df.amount.count_values()
Concatenate |
    select * from df
    union all
    select * from df
    concat(df, df)
Column Type Information |
    select
        column_name,
        data_type,
        character_maximum_length
    from
        information_schema.columns
    where
        table_name = 'df'
    df.dshape
    select
        column_name,
        data_type,
        character_maximum_length
    from
        information_schema.columns
    where
        table_name = 'df'
            and
        column_name = 'amount'
    df.amount.dshape