Jake McCrary

Creating a SQL table with a composite primary key in Clojure

I was interacting with a SQL database using Clojure and needed to create a table so I turned to create-table from clojure.contrib.sql. Looking at the docs for create-table it seemed pretty straight forward. To create a table with columns date, id, symbol, price, and quantity you would write the following.

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(create-table "orders"
              [:date     "date"]
              [:id       "integer"]
              [:symbol   "char(10)"]
              [:price    "integer"]
              [:quantity "integer"])

The above works. I also wanted to specify that columns date and id to form a composite primary key. I wasn’t sure how to specify a composite primary key with create-table and ended up diving into its code.

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(defn create-table
  "Creates a table on the open database connection given a table name and
  specs. Each spec is either a column spec: a vector containing a column
  name and optionally a type and other constraints, or a table-level
  constraint: a vector containing words that express the constraint. All
  words used to describe the table may be supplied as strings or keywords."
  [name & specs]
  (do-commands
   (format "CREATE TABLE %s (%s)"
           (as-str name)
           (apply str 
             (map as-str
              (apply concat 
               (interpose [", "]
                (map (partial interpose " ") specs))))))))

Looking at create-table we can see it creates a SQL statement which is then executed by do-commands. In order to have a composite key we need do-commands to execute a SQL statement that looks similar to below.

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CREATE TABLE track(
  date date,
  id integer,
  symbol char(10),
  price integer,
  quantity integer,
  PRIMARY KEY (date, id)
)

Let’s break down create-table to figure out what we need to pass it to make do-commands run the above statement. The code for create-table is repeated below with comments pointing out what step lines up the code.

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(defn create-table
  [name & specs]
  (do-commands                                              ; step 7
   (format "CREATE TABLE %s (%s)"                           ; step 6
           (as-str name)
           (apply str                                       ; step 5
             (map as-str                                    ; step 4
              (apply concat                                 ; step 3
               (interpose [", "]                            ; step 2
                (map (partial interpose " ") specs))))))))  ; step 1
  1. First create-table takes the sequences in specs and puts a space between each element in each sequence.
  2. The result of step 1 then has a vector containing a comma and a space interposed between each element of it.
  3. concat combined with apply is used to combine each element of the result of step 2 into a single sequence.
  4. as-str (from c.c.string) is mapped over the result of step 3 to make sure every element is a string.
  5. str is used to make one string out of the sequence of strings from step 4.
  6. format is used to substitute in name and the result of step 5 to create the SQL statement.
  7. do-commands executes the statement created in step 6.

Knowing how create-table works now allows us to specify the arguments that will create the orders table with the composite primary key of date and id.

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(create-table "orders"
              [:date     "date"]
              [:id       "integer"]
              [:symbol   "char(10)"]
              [:price    "integer"]
              [:quantity "integer"]
              ["PRIMARY KEY" "(date, id)")
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