Updated documentation

docs/tutorial.md

@@ -582,6 +582,34 @@ db.renderSql(query) ==>
 db.run(query) ==> 154
 ```
 
+ScalaSql also provides `.countBy` and `.countDistinctBy` for more specific counting
+operations. `.countBy` generates `COUNT(column)` and only counts non-null values,
+while `.countDistinctBy` generates `COUNT(DISTINCT column)` to count unique non-null values:
+```scala
+// Count non-null country codes
+val query1 = Country.select.countBy(_.code)
+db.renderSql(query1) ==> "SELECT COUNT(country0.code) AS res FROM country country0"
+
+// Count distinct continents
+val query2 = Country.select.countDistinctBy(_.continent)
+db.renderSql(query2) ==> "SELECT COUNT(DISTINCT country0.continent) AS res FROM country country0"
+
+db.run(query2) ==> 7  // 7 distinct continents in the database
+```
+
+You can also use `.count` and `.countDistinct` on mapped expressions:
+```scala
+// Count non-null population values after mapping
+val query3 = Country.select.map(_.population).count
+db.renderSql(query3) ==> "SELECT COUNT(country0.population) AS res FROM country country0"
+
+// Count distinct population density categories
+val query4 = Country.select
+  .map(c => c.population / c.surfaceArea)
+  .countDistinct
+db.renderSql(query4) ==> "SELECT COUNT(DISTINCT (country0.population / country0.surfacearea)) AS res FROM country country0"
+```
+
 If you want to perform multiple aggregates at once, you can use the `.aggregate`
 function. Below, we run a single query that returns the minimum, average, and
 maximum populations across all countries in our dataset
@@ -599,6 +627,25 @@ FROM country country0
 db.run(query) ==> (0, 25434098, 1277558000)
 ```
 
+You can combine COUNT operations with other aggregates in the same query:
+```scala
+val query = Country.select
+  .aggregate(cs => (
+    cs.countBy(_.population),        // Count non-null populations
+    cs.countDistinctBy(_.continent), // Count distinct continents
+    cs.sumBy(_.population)           // Sum all populations
+  ))
+db.renderSql(query) ==> """
+SELECT
+  COUNT(country0.population) AS res_0,
+  COUNT(DISTINCT country0.continent) AS res_1,
+  SUM(country0.population) AS res_2
+FROM country country0
+"""
+
+db.run(query) ==> (239, 7, 6078749450)
+```
+
 ### Sort/Drop/Take
 
 You can use `.sortBy` to order the returned rows, and `.drop` and `.take`