Tool of Thought

APL for the Practical Man

Anatomy of a Query, Part 2

December 20, 2022

In the first part of this series, Anatomy of a Query, we looked at how a query is an ordered collection of names and expressions, divided into various sections. For simplicity, we assumed all of the data was in a single table. Of course data may be in multiple tables, or even multiple databases, and perhaps in a CSV file hanging around on the disk, not even in the database. To explore how FlipDB handles multiple tables, we will refer to CJ Date's classic Suppliers and Parts database:

 ── SandP.S ──────────────────────────────────            
  ┌SNO────┐  ┌STATUS┐  ┌CITY────┐  ┌SNAME───┐             
  ↓S1     │  ↓20    │  ↓London  │  ↓Smith   │             
  │S2     │  │10    │  │Paris   │  │Jones   │             
  │S3     │  │30    │  │Paris   │  │Blake   │             
  │S4     │  │20    │  │London  │  │Clark   │             
  │S5     │  │30    │  │Athens  │  │Adams   │             
  └Char(2)┘  └Int8──┘  └Char(10)┘  └Char(10)┘             
 ── 5 rows by 4 columns ──────────────────────            
 ── SandP.P ────────────────────────────────────────────  
  ┌PNO────┐  ┌PNAME──┐  ┌COLOR──┐  ┌WEIGHT┐  ┌CITY────┐   
  ↓P1     │  ↓Nut    │  ↓Red    │  ↓12    │  ↓London  │   
  │P2     │  │Bolt   │  │Green  │  │17    │  │Paris   │   
  │P3     │  │Screw  │  │Blue   │  │17    │  │Oslo    │   
  │P4     │  │Screw  │  │Red    │  │14    │  │London  │   
  │P5     │  │Cam    │  │Blue   │  │12    │  │Paris   │   
  │P6     │  │Cog    │  │Red    │  │19    │  │London  │   
  └Char(2)┘  └Char(5)┘  └Char(5)┘  └Int8──┘  └Char(10)┘   
 ── 6 rows by 5 columns ────────────────────────────────  
 ── SandP.SP ──────────────────────────────────────────── 
  ┌AUTOKEY┐  ┌QTY──┐  ┌SDATE─────┐  ┌SNO────┐  ┌PNO────┐  
  ↓1      │  ↓300  │  ↓2008-10-27│  ↓S1     │  ↓P1     │  
  │2      │  │200  │  │2008-08-19│  │S1     │  │P2     │  
  │3      │  │400  │  │2008-05-18│  │S1     │  │P3     │  
  │4      │  │200  │  │2007-08-18│  │S1     │  │P4     │  
  │5      │  │100  │  │2007-08-17│  │S1     │  │P5     │  
  │6      │  │100  │  │2009-12-16│  │S1     │  │P6     │  
  │7      │  │300  │  │2009-07-31│  │S2     │  │P1     │  
  │8      │  │400  │  │2009-12-17│  │S2     │  │P2     │  
  │9      │  │200  │  │2009-07-24│  │S3     │  │P2     │  
  │10     │  │200  │  │2009-10-03│  │S4     │  │P2     │  
  │11     │  │300  │  │2009-06-28│  │S4     │  │P4     │  
  │12     │  │400  │  │2009-11-04│  │S4     │  │P5     │  
  └Int8───┘  └Int16┘  └Date──────┘  └Char(2)┘  └Char(2)┘  
 ── 12 rows by 5 columns ──────────────────────────────── 

A FlipDB query generally starts with a certain table, called the starting table. Column names in this table are specified directly; there is no need to qualify them. If starting in the SP table, for example, we may specify the quantity column simply as:


In SQL, in order to query data across multiple tables, the tables must be joined in some way. Then columns and rows are selected from this combined table or view. For example:

select distinct sp.pno,sname from
supplier inner join sp using (sno)
where = 'London';

This gets complicated very quickly.

Because a FlipDB query is an ordered set of names and expressions, we may directly specify a column from another table in an expression, using a number of different syntaxes and special functions.

If we are lucky enough to have a formal database set up, with primary, alternate, and foreign keys, we can use a simple dot syntax to specify a column in a different table via a foreign key. Starting with the SP table, we can reach into the P table and get the PNAME column:


This is called a simple join. Note there is no explicit reference to the foreign table P. The foreign key column PNO implies it. The result is a simple column, as by definition there is a one-to-many relationship between tables S and SP. The column PNAME will be sorted and replicated to line up up with the SP table. A simple join can link through multiple tables, by specifying multiple foreign keys:


We can use foreign keys to go the other way as well. Starting in the P table we can reference a column in the SP table as follows:


Here we make specific refence to the SP table, indicate in brackets the foreign key column in SP that points back to our primary table P, and finally after the dot, the name of the column in SP to materialize. By definition, this value is a partitioned column, as there is a many-to-one relationship between tables SP and P. This is called a partition join.

Simple and partition joins may be combined to link through multiple tables:


If we don't have a foreign key specified, we can do explicit look-ups:


This materializes ColC from Table2, by looking up the values of ColA from the starting table in ColB from table 2. If the ColA-ColB relationship is one-to-one, or many to one, the result is simple, while if the relationship is one-to-many or many-to-many, the result is partitioned. This is called a general join.

Sometimes we want to access a column in another table without regard to any key:

SP[].QTY                                         `

Here the result is enclosed as it is non-conforming; there is no correlation between the starting table and this column. This is useful as the where clause has no effect. We can reach out of the context we are in and grab an entire column from another table, or even the current table. This is called an enclosed join.

Directly accessing data from other tables is supremely useful because we want to apply APL-style solutions directly in our queries, avoiding the headache of SQL. Consider a query starting in the parts table P, with the following select clause:

InParisany 'Paris' eq SupplierIn
IsCompetitionnot unique SupplierIn

Since we are starting in the parts table, we can access the part number PNO directly. We then foreign-key hop, first in the reverse, one-to-many direction into the suppliers and parts table SP, and then in the many-to-one direction into the suppliers table to get the CITY column. This yields a partitioned column showing the city for each supplier that supplies that part. We can then use this to answer a myriad of questions. For example, what parts are supplied by suppliers located in Paris? Or what parts a supplied competitively, that is by two or more suppliers in the same city? This gives us the following table:


Of course these last two Boolean expressions would most likely be placed in the where or having clause, we just show them here in the select clause for explanatory purposes.

In Part 3 of this series we will look at additional ways to get data from other tables, using functions rather than column join syntax, that avoid the need to have formally specified keys.