## Overview

The question gets asked how to build dynamic or composable queries in postmodern. First we need to understand the context - is the programmer building the query or are you taking data from a user and using that to build a query? We need to remember that the query macro assumes that everything that is not a list starting with a keyword will evaluate to a string.

In any case you will need to ensure that either you have control over the inputs or they still result in parameterized queries. If not you have opened yourself up to an sql injection attack.

If you are not using s-sql, then it becomes easy. The query macro assumes that everything that is not a list starting with a keyword will evaluate to a string. That means you can build it with a simple format string

    (query (format nil "select ~a from ~a where ~a"  "carrots" "garden" "length > 3"))

With s-sql, there are generally three approaches to building dynamic or composible queries: pass symbols and values as variables, use sql-compile or use :raw.

For purposes of this example, we will use the following employee table:

    (query (:create-table employee ((id :type int)
                                    (name :type text)
                                    (salary :type numeric)
                                    (start_date :type date)
                                    (city :type text)
                                    (region :type char)
                                    (age :type int))))

    (query (:insert-rows-into 'employee
                              :columns 'id 'name 'salary 'start-date 'city 'region 'age
                              :values '((1 "Jason" 40420 "02/01/94" "New York" "W" 29)
                                        (2 "Robert" 14420 "01/02/95" "Vancouver" "N" 21)
                                        (3 "Celia" 24020 "12/03/96" "Toronto" "W" 24)
                                        (4 "Linda" 40620 "11/04/97" "New York" "N" 28)
                                        (5 "David" 80026 "10/05/98" "Vancouver" "W" 31)
                                        (6 "James" 70060 "09/06/99" "Toronto" "N" 26)
                                        (7 "Alison" 90620 "08/07/00" "New York" "W" 38)
                                        (8 "Chris" 26020 "07/08/01" "Vancouver" "N" 22)
                                        (9 "Mary" 60020 "06/08/02" "Toronto" "W" 34))))

### Approach #1 Using symbols in variables

#### Select Statements

Consider the following two toy examples where we determine the table and columns to be selected using symbols (either keyword or quoted) inside variables.

    (let ((table 'employee) (col1 :id) (col2 :name) (id 3))
        (query (:select col1 col2 :from table :where (:= 'id '$1)) id))
    ((3 "Celia"))

    (let ((table 'employee) (col1 'name) (col2 'salary) (id 3))
        (query (:select col1 col2 :from table :where (:= 'id '$1)) id))
    (("Celia" 24020))

This will not work if you use strings instead of symbols because sql-expand will wrap the strings in the variables in escape format as if they were string constants and Postgresql will throw an error because it is not expecting string constants in the middle of a select statement.

#### Update Statements

This works with update statements as well

    (let ((table 'employee) (col1 :id) (col2 :name) (new-name "Celeste") (id 3))
        (query (:update table :set col2 new-name :where (:= col1 '$1)) id)
        (query (:select col1 col2 :from table :where (:= 'id '$1)) id))
    ((3 "Celeste"))

#### Insert Statements

This works with insert-into statements as well

    (let ((table 'employee) (col1 'id) (col2 'name) (new-name "Rochelle")
          (id 10) (col3 'salary) (col3-value 3452) (col4 'start-date)
          (col4-value "02/01/03") (col5 'city) (col5-value "Victoria")
          (col6 'region) (col6-value "N") (col7 'age) (col7-value 32))
      (query (:insert-into table :set col1 id col2 new-name col3 col3-value
                           col4 col4-value col5 col5-value col6 col6-value
                           col7 col7-value)))

    (query (:select 'id 'name 'salary :from 'employee :where (:= 'id 10 )))
    ((10 "Rochelle" 3452))

#### Delete Statements

This works with delete statements as well

    (let ((table 'employee) (col1 :id) (col1-value 10))
       (query (:delete-from table :where (:= col1 col1-value))))

### Approach #2 Use sql-compile

Sql-compile does a run-time compilation of an s-sql expression. In the following example, we create a function that accepts a where-clause, a table-name, 3 columns to select and two parameters to go into the where clause.

    (defun toy-example (where-clause table-name col1 col2 col3 arg1 arg2)
      (with-test-connection
       (query (sql-compile
               (append `(:select ,col1 ,col2 ,col3 :from ,table-name :where)
                       where-clause))
              arg1 arg2)))

    (toy-example '((:and (:= 'city '$1) (:> 'salary '$2))) 'employee 'id 'name 'city "Toronto" 45000)

    ((6 "James" "Toronto") (9 "Mary" "Toronto"))

If we just look at what this call to sql-compile in toy-example generates, it would look like:

    "(SELECT id, name, city FROM employee WHERE ((city = $1) and (salary > $2)))"

This example is still a parameterized query but for security reasons you will need to be very careful how you generate the where clause.

Another example with sql-compile and append, in this case updating a table and setting two columns to NULL.

    (sql-compile (append '(:update :table1 :set)
                         (loop for a in '("col1" "col2")
                               collect a
                               collect :NULL)))

    "UPDATE table1 SET E'col1' = NULL, E'col2' = NULL"

Lets think about it differently. What if we know the universe of columns we want to select, but want to conditionally select some of them. Suppose we know our targetted table has columns:

'id 'name 'salary 'start-date 'city 'region 'age.

We may decide we always want name, city and age, but salary and start-date are conditional.

    (defun toy-example-2 (salaryp start-date-p)
      (sql-compile
       (remove nil `(:select 'name 'city 'age
                             ,(if salaryp 'salary nil)
                             ,(if start-date-p 'start-date nil)
                             :from 'employee))))

    (query (toy-example-2 t t))

    (("Jason" "New York" 29 40420 #<SIMPLE-DATE:DATE 01-02-1994>)
     ("Robert" "Vancouver" 21 14420 #<SIMPLE-DATE:DATE 02-01-1995>)
     ("Celia" "Toronto" 24 24020 #<SIMPLE-DATE:DATE 03-12-1996>)
     ("Linda" "New York" 28 40620 #<SIMPLE-DATE:DATE 04-11-1997>)
     ("David" "Vancouver" 31 80026 #<SIMPLE-DATE:DATE 05-10-1998>)
     ("James" "Toronto" 26 70060 #<SIMPLE-DATE:DATE 06-09-1999>)
     ("Alison" "New York" 38 90620 #<SIMPLE-DATE:DATE 07-08-2000>)
     ("Chris" "Vancouver" 22 26020 #<SIMPLE-DATE:DATE 08-07-2001>)
     ("Mary" "Toronto" 34 60020 #<SIMPLE-DATE:DATE 08-06-2002>))

    (query (toy-example-2 t nil))

    (("Jason" "New York" 29 40420) ("Robert" "Vancouver" 21 14420)
     ("Celia" "Toronto" 24 24020) ("Linda" "New York" 28 40620)
     ("David" "Vancouver" 31 80026) ("James" "Toronto" 26 70060)
     ("Alison" "New York" 38 90620) ("Chris" "Vancouver" 22 26020)
     ("Mary" "Toronto" 34 60020))

You could skip the (remove nil… portion and substitute t for nil. E.g.

    (defun toy-example-2 (salaryp start-date-p)
      (sql-compile
       `(:select 'name 'city 'age
                 ,(if salaryp 'salary t)
                 ,(if start-date-p 'start-date t)
                 :from 'employee)))

But I prefer to remove those segments completely from the query.

Following on this same thread of thought, you can define a portion of the sql in a let clause:

    (let ((sql1 '(:= name "Jason")))
      (query (sql-compile
        `(:select 'name 'city 'age :from 'employee :where ,sql1))))

    (("Jason" "New York" 29))

### Approach #3 Use :raw

To quote Marijn, the :raw keyword takes a string and inserts it straight into the query. I try to stay away from :raw if possible, but sometimes…

    (query (:select (:raw "tmp1.name") :from (:as 'baz (:raw "tmp1"))))