Define ER Model and ER Diagram

<div class="infographic-erd-example">
  <style>
    .infographic-erd-example {
      --ink:        #1a1a1a;
      --ink-soft:   #5c5c5c;
      --paper:      #ffffff;
      --paper-alt:  #f0ece4;
      --accent:     #2563eb;
      --accent-soft:#e3ebfd;
      --line:       #ddd6c9;
      --prime:      #c0392b;
      --mono-font:  ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;

      font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif;
      background: var(--paper);
      color: var(--ink);
      padding: 2rem 1.5rem 2.5rem;
      max-width: 820px;
      margin: 0 auto;
      box-sizing: border-box;
    }
    .infographic-erd-example[data-theme="dark"] {
      --ink:        #e8e6e1;
      --ink-soft:   #9a968c;
      --paper:      #121212;
      --paper-alt:  #1e1e26;
      --accent:     #5b8def;
      --accent-soft:#202942;
      --line:       #2c313d;
      --prime:      #e57373;
    }
    .infographic-erd-example *, .infographic-erd-example *::before, .infographic-erd-example *::after {
      box-sizing: border-box;
    }
    .infographic-erd-example h2 {
      font-size: 1.4rem;
      margin: 0 0 0.25rem;
      letter-spacing: -0.01em;
    }
    .infographic-erd-example .subtitle {
      color: var(--ink-soft);
      font-size: 0.92rem;
      margin: 0 0 1.5rem;
      max-width: 56ch;
    }

    .infographic-erd-example .diagram-wrap {
      background: var(--paper-alt);
      border-radius: 14px;
      padding: 0.75rem 0.5rem 0.25rem;
      margin-bottom: 1rem;
    }
    .infographic-erd-example svg {
      width: 100%;
      height: auto;
      display: block;
    }
    .infographic-erd-example .shape {
      fill: var(--paper);
      stroke: var(--ink);
      stroke-width: 2;
      cursor: pointer;
      transition: stroke 0.25s ease, stroke-width 0.25s ease, filter 0.25s ease;
    }
    .infographic-erd-example .shape.is-active {
      stroke: var(--accent);
      stroke-width: 3;
      filter: drop-shadow(0 0 4px var(--accent-soft));
    }
    .infographic-erd-example .conn-line {
      stroke: var(--ink-soft);
      stroke-width: 1.6;
      transition: stroke 0.25s ease, stroke-width 0.25s ease;
    }
    .infographic-erd-example .conn-line.is-active {
      stroke: var(--accent);
      stroke-width: 2.4;
    }
    .infographic-erd-example .node-label {
      font-size: 13px;
      font-weight: 700;
      fill: var(--ink);
      text-anchor: middle;
      dominant-baseline: central;
      pointer-events: none;
    }
    .infographic-erd-example .node-label.prime-text {
      fill: var(--prime);
      text-decoration: underline;
    }
    .infographic-erd-example .kicker-text {
      font-size: 9.5px;
      letter-spacing: 0.04em;
      fill: var(--ink-soft);
      text-anchor: middle;
      text-transform: uppercase;
      pointer-events: none;
    }
    .infographic-erd-example .node-group { cursor: pointer; }

    .infographic-erd-example .info-panel {
      border-radius: 12px;
      border: 1px solid var(--line);
      background: var(--paper);
      padding: 0.85rem 1rem;
      margin-bottom: 0.6rem;
    }
    .infographic-erd-example .info-role {
      display: inline-block;
      font-size: 0.66rem;
      font-weight: 700;
      text-transform: uppercase;
      letter-spacing: 0.06em;
      color: var(--accent);
      background: var(--accent-soft);
      padding: 0.15rem 0.55rem;
      border-radius: 999px;
      margin-bottom: 0.4rem;
    }
    .infographic-erd-example .info-name {
      font-weight: 700;
      font-size: 1rem;
      margin: 0 0 0.25rem;
    }
    .infographic-erd-example .info-desc {
      font-size: 0.85rem;
      color: var(--ink-soft);
      margin: 0;
    }

    .infographic-erd-example .progress-track {
      height: 3px;
      background: var(--line);
      border-radius: 999px;
      overflow: hidden;
      margin-bottom: 1rem;
    }
    .infographic-erd-example .progress-fill {
      height: 100%;
      width: 0%;
      background: var(--accent);
    }
    .infographic-erd-example .progress-fill.is-animating {
      animation: erd-progress 2.4s linear forwards;
    }
    @keyframes erd-progress {
      from { width: 0%; }
      to { width: 100%; }
    }
    .infographic-erd-example .hint {
      font-size: 0.72rem;
      color: var(--ink-soft);
      text-align: center;
      margin: -0.6rem 0 1rem;
    }

    .infographic-erd-example .takeaway {
      margin-top: 0.5rem;
      font-size: 0.88rem;
      color: var(--ink-soft);
      border-left: 3px solid var(--accent);
      padding: 0.5rem 0 0.5rem 0.9rem;
    }
    .infographic-erd-example .takeaway strong { color: var(--ink); }

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      .infographic-erd-example *, .infographic-erd-example *::before, .infographic-erd-example *::after {
        animation-duration: 0.001ms !important;
        transition-duration: 0.001ms !important;
      }
    }
  </style>

  <h2>Entity Relationship Diagram (ERD)</h2>
  <p class="subtitle">A full ERD, piece by piece. It walks itself — or click any shape to jump straight to it.</p>

  <div class="diagram-wrap">
    <svg id="erd-svg" viewBox="0 0 700 380" xmlns="http://www.w3.org/2000/svg">
      <!-- connecting lines -->
      <line class="conn-line" id="ln-name-student" x1="210" y1="86" x2="225" y2="165"/>
      <line class="conn-line" id="ln-email-student" x1="210" y1="294" x2="225" y2="215"/>
      <line class="conn-line" id="ln-ssn-student" x1="120" y1="190" x2="170" y2="190"/>
      <line class="conn-line" id="ln-student-joins" x1="280" y1="190" x2="305" y2="190"/>
      <line class="conn-line" id="ln-joins-course" x1="395" y1="190" x2="420" y2="190"/>
      <line class="conn-line" id="ln-cid-course" x1="580" y1="190" x2="530" y2="190"/>
      <line class="conn-line" id="ln-title-course" x1="490" y1="86" x2="475" y2="165"/>
      <line class="conn-line" id="ln-duration-course" x1="490" y1="294" x2="475" y2="215"/>

      <!-- Name attribute -->
      <g class="node-group" data-key="name">
        <text class="kicker-text" x="210" y="26">Attribute</text>
        <ellipse class="shape" cx="210" cy="60" rx="56" ry="26"/>
        <text class="node-label" x="210" y="60">Name</text>
      </g>

      <!-- Title attribute -->
      <g class="node-group" data-key="title">
        <text class="kicker-text" x="490" y="26">Attribute</text>
        <ellipse class="shape" cx="490" cy="60" rx="56" ry="26"/>
        <text class="node-label" x="490" y="60">Title</text>
      </g>

      <!-- SSN prime attribute -->
      <g class="node-group" data-key="ssn">
        <text class="kicker-text" x="70" y="158">Prime Attribute</text>
        <ellipse class="shape" cx="70" cy="190" rx="50" ry="24"/>
        <text class="node-label prime-text" x="70" y="190">SSN</text>
      </g>

      <!-- CID prime attribute -->
      <g class="node-group" data-key="cid">
        <text class="kicker-text" x="630" y="158">Prime Attribute</text>
        <ellipse class="shape" cx="630" cy="190" rx="50" ry="24"/>
        <text class="node-label prime-text" x="630" y="190">CID</text>
      </g>

      <!-- Student entity -->
      <g class="node-group" data-key="student">
        <text class="kicker-text" x="225" y="153">Entity</text>
        <rect class="shape" x="170" y="165" width="110" height="50" rx="2"/>
        <text class="node-label" x="225" y="190">Student</text>
      </g>

      <!-- Course entity -->
      <g class="node-group" data-key="course">
        <text class="kicker-text" x="475" y="153">Entity</text>
        <rect class="shape" x="420" y="165" width="110" height="50" rx="2"/>
        <text class="node-label" x="475" y="190">Course</text>
      </g>

      <!-- Joins relationship -->
      <g class="node-group" data-key="joins">
        <text class="kicker-text" x="350" y="140">Relationship</text>
        <polygon class="shape" points="350,158 395,190 350,222 305,190"/>
        <text class="node-label" x="350" y="190">Joins</text>
      </g>

      <!-- Email attribute -->
      <g class="node-group" data-key="email">
        <ellipse class="shape" cx="210" cy="320" rx="56" ry="26"/>
        <text class="node-label" x="210" y="320">Email</text>
        <text class="kicker-text" x="210" y="360">Attribute</text>
      </g>

      <!-- Duration attribute -->
      <g class="node-group" data-key="duration">
        <ellipse class="shape" cx="490" cy="320" rx="56" ry="26"/>
        <text class="node-label" x="490" y="320">Duration</text>
        <text class="kicker-text" x="490" y="360">Attribute</text>
      </g>
    </svg>
  </div>

  <div class="progress-track"><div class="progress-fill" id="erd-progress"></div></div>

  <div class="info-panel">
    <span class="info-role" id="erd-role">Entity</span>
    <p class="info-name" id="erd-name">Student</p>
    <p class="info-desc" id="erd-desc"></p>
  </div>
  <div class="hint" id="erd-hint">auto-touring · click any shape to pause &amp; jump there</div>

  <p class="takeaway"><strong>Six shapes, one story:</strong> two entities, their attributes, and the relationship that ties them together — every ERD is this same pattern, just repeated.</p>

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        student: {
          role: 'Entity', name: 'Student',
          desc: 'A rectangle — the entity itself. Every attribute and relationship around it exists to describe or connect to Student.',
          lines: ['ln-name-student', 'ln-email-student', 'ln-ssn-student', 'ln-student-joins']
        },
        ssn: {
          role: 'Prime Attribute', name: 'SSN',
          desc: 'Underlined in red — SSN uniquely identifies each Student. It\'s the primary key of the entity.',
          lines: ['ln-ssn-student']
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          role: 'Attribute', name: 'Name',
          desc: 'A regular attribute of Student — it describes the entity but can\'t uniquely identify it on its own.',
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        },
        email: {
          role: 'Attribute', name: 'Email',
          desc: 'Another attribute of Student, connected the same way as Name.',
          lines: ['ln-email-student']
        },
        joins: {
          role: 'Relationship', name: 'Joins',
          desc: 'The diamond — this is what actually connects Student to Course. Without it, they\'re just two unrelated entities.',
          lines: ['ln-student-joins', 'ln-joins-course']
        },
        course: {
          role: 'Entity', name: 'Course',
          desc: 'The second entity in this diagram, linked to Student only through the Joins relationship.',
          lines: ['ln-title-course', 'ln-duration-course', 'ln-cid-course', 'ln-joins-course']
        },
        cid: {
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          desc: 'Course\'s primary key — uniquely identifies each Course, the same role SSN plays for Student.',
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        title: {
          role: 'Attribute', name: 'Title',
          desc: 'A regular attribute describing the Course.',
          lines: ['ln-title-course']
        },
        duration: {
          role: 'Attribute', name: 'Duration',
          desc: 'Another attribute of Course, alongside Title.',
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        }
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        hint.textContent = 'auto-touring · click any shape to pause & jump there';
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</div>

.demo {
  font-family: system-ui, sans-serif;
  padding: 16px;
}

.demo h1 {
  color: #2563eb;
  margin: 0 0 8px;
}

.demo p {
  color: #4b5563;
  margin: 0;
}

document.querySelector('.demo h1')?.addEventListener('click', () => {
  alert('Hello from live preview!');
});