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skills/optimizing-query-performance/references/field-selection.md

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+# Field Selection Guide
+
+## Select vs Include
+
+**select:** Choose specific fields (excludes all others)
+
+```typescript
+const user = await prisma.user.findUnique({
+  where: { id: 1 },
+  select: {
+    id: true,
+    email: true
+  }
+})
+```
+
+**include:** Add relations to default fields
+
+```typescript
+const user = await prisma.user.findUnique({
+  where: { id: 1 },
+  include: {
+    orders: true
+  }
+})
+```
+
+**Cannot use both select and include in same query.**
+
+## Nested Selection
+
+```typescript
+const users = await prisma.user.findMany({
+  select: {
+    id: true,
+    email: true,
+    orders: {
+      select: {
+        id: true,
+        total: true,
+        createdAt: true
+      },
+      where: { status: 'completed' },
+      orderBy: { createdAt: 'desc' },
+      take: 5
+    }
+  }
+})
+```
+
+Only fetches recent completed orders, not all orders.
+
+## Counting Relations Without Loading
+
+```typescript
+const users = await prisma.user.findMany({
+  select: {
+    id: true,
+    email: true,
+    _count: {
+      select: {
+        orders: true,
+        posts: true
+      }
+    }
+  }
+})
+```
+
+Returns counts without loading actual relation records.