add sql driver example

src/executor.sql.test.js

@@ -0,0 +1,47 @@
+describe('executor', () => {
+  it('sql driver', () => {
+    const someNativeDriver = {
+      execQry (qry) {
+        return `exec query ${qry.trim()}`
+      },
+      execNonQry(nonQry) {
+        return `exec non query ${nonQry.trim()}`
+      }
+    }
+
+    const makeEntity = (table) => {
+      let qry = '';
+      return {
+        $() {
+          let q = qry;
+          qry = '';
+          return someNativeDriver.execQry(q)
+        },
+        $$() {
+          let q = qry;
+          qry = '';
+          return someNativeDriver.execNonQry(q)
+        },
+        select(cols) {
+          const projection = typeof cols === 'string' ? cols : cols.join(', ')
+          qry += `SELECT ${projection} from ${table}
+`;
+          return this;
+        },
+        where(condition) {
+          qry +=
+`WHERE ${condition}
+`;
+          return this;
+        }
+      }
+    }
+    const sqlDriver = {
+      user: makeEntity('user')
+    }
+
+    const rs = sqlDriver.user.select(['fullName', 'username']).where('age > 10').$()
+    expect(rs).toBe('exec query SELECT fullName, username from user\n' +
+        'WHERE age > 10')
+  })
+})