feat: Added support for DISTINCT queries (typeorm#4109)

src/query-builder/QueryExpressionMap.ts

@@ -46,6 +46,11 @@ export class QueryExpressionMap {
      */
     selects: SelectQuery[] = [];
 
+    /**
+     * Whether SELECT is DISTINCT.
+     */
+    selectDistinct: boolean = false;
+
     /**
      * FROM-s to be selected.
      */

src/query-builder/SelectQueryBuilder.ts

@@ -158,6 +158,14 @@ export class SelectQueryBuilder<Entity> extends QueryBuilder<Entity> implements
         return this;
     }
 
+    /**
+     * Sets whether the selection is DISTINCT.
+     */
+    distinct(distinct: boolean = true): this {
+        this.expressionMap.selectDistinct = distinct;
+        return this;
+    }
+
     /**
      * Specifies FROM which entity's table select/update/delete will be executed.
      * Also sets a main string alias of the selection data.
@@ -1400,8 +1408,9 @@ export class SelectQueryBuilder<Entity> extends QueryBuilder<Entity> implements
 
                 return this.getTableName(alias.tablePath!) + " " + this.escape(alias.name);
             });
+        const select = "SELECT " + (this.expressionMap.selectDistinct ? "DISTINCT " : "");
         const selection = allSelects.map(select => select.selection + (select.aliasName ? " AS " + this.escape(select.aliasName) : "")).join(", ");
-        return "SELECT " + selection + " FROM " + froms.join(", ") + lock;
+        return select + selection + " FROM " + froms.join(", ") + lock;
     }
 
     /**

test/functional/query-builder/select/query-builder-select.ts

@@ -27,6 +27,21 @@ describe("query builder > select", () => {
             "FROM post post");
     })));
 
+    it("should append all entity mapped columns from main selection to SELECT DISTINCT statement", () => Promise.all(connections.map(async connection => {
+        const sql = connection.manager.createQueryBuilder(Post, "post")
+            .distinct()
+            .disableEscaping()
+            .getSql();
+
+        expect(sql).to.equal("SELECT DISTINCT post.id AS post_id, " +
+            "post.title AS post_title, " +
+            "post.description AS post_description, " +
+            "post.rating AS post_rating, " +
+            "post.version AS post_version, " +
+            "post.categoryId AS post_categoryId " +
+            "FROM post post");
+    })));
+
     it("should append all entity mapped columns from both main selection and join selections to select statement", () => Promise.all(connections.map(async connection => {
         const sql = connection.createQueryBuilder(Post, "post")
             .leftJoinAndSelect("category", "category")