Primary key

<div class="demo"><div class="head"><b>PRIMARY KEY (id)</b><span>users</span></div><div class="stage pk"><div class="existing"><div class="cell">1 · Ana</div><div class="cell">3 · Bo</div></div><div class="arrow">←</div><div class="cell incoming" id="incoming">3 · Cy</div></div><div class="foot"><span id="verdict">duplicate id: rejected</span><span>unique + not null</span></div></div>

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let n=0;function tick(){let e=document.getElementById('incoming');e.textContent=n?'4 · Cy':'3 · Cy';e.classList.toggle('bad',!n);e.classList.toggle('on',!!n);document.getElementById('verdict').textContent=n?'id 4: inserted':'duplicate id: rejected';n=1-n}tick();const t=setInterval(tick,1100);document.querySelector('.demo').onclick=()=>{clearInterval(t);tick()}