fixes #547 by adding examples

Signed-off-by: Sean Corfield <sean@corfield.org>

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Changes
 
 * 2.6.next in progress
+  * Address [#547](https://github.com/seancorfield/honeysql/issues/547) by adding examples of conditional SQL building with the helpers to the README and the `honey.sql.helpers` ns docstring.
   * Performance optimizations via PRs [#545](https://github.com/seancorfield/honeysql/pull/545) and [#546](https://github.com/seancorfield/honeysql/pull/546) [@alexander-yakushev](https://github.com/alexander-yakushev).
   * Address [#544](https://github.com/seancorfield/honeysql/issues/544) by adding support for MySQL's `VALUES ROW(..)` syntax.
   * Fix [#543](https://github.com/seancorfield/honeysql/issues/543) by supporting both symbols and keywords in named parameters.

README.md

@@ -215,6 +215,24 @@ If you want to replace a clause, you can `dissoc` the existing clause first, sin
 => ["SELECT * FROM foo WHERE (a = ?) AND (b < ?)" 1 100]
 ```
 
+The power of this approach comes from the abiliity to programmatically and
+conditionally build up queries:
+
+<!-- :test-doc-blocks/skip -->
+```clojure
+(defn fetch-user [& {:keys [id name]}]
+  (-> (select :*)
+      (from :users)
+      (cond->
+        id    (where [:= :id id])
+        name  (where [:= :name name]))
+      sql/format))
+```
+
+You can call `fetch-user` with either `:id` or `:name` _or both_ and get back
+a query with the appropriate `WHERE` clause, since the helpers will merge the
+conditions into the query DSL.
+
 Column and table names may be aliased by using a vector pair of the original
 name and the desired alias:
 
@@ -226,6 +244,21 @@ name and the desired alias:
 => ["SELECT a, b AS bar, c, d AS x FROM foo AS quux WHERE (quux.a = ?) AND (bar < ?)" 1 100]
 ```
 
+or conditionally:
+
+<!-- :test-doc-blocks/skip -->
+```clojure
+(-> (select :a [:b :bar])
+    (cond->
+      need-c (select :c)
+      x-val  (select [:d :x]))
+    (from [:foo :quux])
+    (where [:= :quux.a 1] [:< :bar 100])
+    (cond->
+      x-val  (where [:> :x x-val]))
+    sql/format)
+```
+
 In particular, note that `(select [:a :b])` means `SELECT a AS b` rather than
 `SELECT a, b` -- helpers like `select` are generally variadic and do not take
 a collection of column names.

src/honey/sql/helpers.cljc

@@ -17,6 +17,16 @@
       (sql/format))
 ```
 
+  or conditionally like this:
+
+```
+  (-> (select :a :b :c)
+      (from :table)
+      (cond->
+        id (where [:= :id id]))
+      (sql/format))
+```
+
   Therefore all helpers can take an existing DSL expression
   as their first argument or, if the first argument is not
   a hash map, an empty DSL is assumed -- an empty hash map.