diff --git a/keyserver/src/fetchers/message-fetchers.js b/keyserver/src/fetchers/message-fetchers.js
--- a/keyserver/src/fetchers/message-fetchers.js
+++ b/keyserver/src/fetchers/message-fetchers.js
@@ -108,14 +108,25 @@
     LEFT JOIN messages m ON m.id = n.message
     LEFT JOIN uploads up
       ON m.type IN (${[messageTypes.IMAGES, messageTypes.MULTIMEDIA]})
-        AND JSON_CONTAINS(m.content, CAST(up.id as JSON), '$')
+  `;
+  const dbType = await getDBType();
+  if (dbType === 'mysql5.7') {
+    collapseQuery.append(SQL`
+        AND JSON_CONTAINS(m.content, CAST(up.id AS JSON), '$')
+    `);
+  } else {
+    collapseQuery.append(SQL`
+        AND JSON_CONTAINS(m.content, up.id, '$')
+    `);
+  }
+  collapseQuery.append(SQL`
     LEFT JOIN memberships mm ON mm.thread = m.thread AND mm.user = n.user
     LEFT JOIN memberships stm
       ON m.type = ${messageTypes.CREATE_SUB_THREAD}
         AND stm.thread = m.content AND stm.user = n.user
     WHERE n.rescinded = 0 AND
       JSON_EXTRACT(mm.permissions, ${visibleExtractString}) IS TRUE AND
-  `;
+  `);
   collapseQuery.append(mergeOrConditions(sqlTuples));
   collapseQuery.append(SQL`ORDER BY m.time DESC`);
   const [collapseResult] = await dbQuery(collapseQuery);
@@ -298,13 +309,24 @@
         FROM messages m
         LEFT JOIN uploads up
           ON m.type IN (${[messageTypes.IMAGES, messageTypes.MULTIMEDIA]})
-            AND JSON_CONTAINS(m.content, CAST(up.id as JSON), '$')
+  `;
+  const dbType = await getDBType();
+  if (dbType === 'mysql5.7') {
+    query.append(SQL`
+            AND JSON_CONTAINS(m.content, CAST(up.id AS JSON), '$')
+    `);
+  } else {
+    query.append(SQL`
+            AND JSON_CONTAINS(m.content, up.id, '$')
+    `);
+  }
+  query.append(SQL`
         LEFT JOIN memberships mm
           ON mm.thread = m.thread AND mm.user = ${viewerID}
         LEFT JOIN memberships stm ON m.type = ${messageTypes.CREATE_SUB_THREAD}
           AND stm.thread = m.content AND stm.user = ${viewerID}
         WHERE JSON_EXTRACT(mm.permissions, ${visibleExtractString}) IS TRUE AND
-  `;
+  `);
   query.append(selectionClause);
   // - We specify a ridiculously high LIMIT here to force MariaDB to maintain
   //   the ORDER BY. Otherwise the query optimizer will assume that the order
@@ -528,12 +550,23 @@
     FROM messages m
     LEFT JOIN uploads up
       ON m.type IN (${[messageTypes.IMAGES, messageTypes.MULTIMEDIA]})
-        AND JSON_CONTAINS(m.content, CAST(up.id as JSON), '$')
+  `;
+  const dbType = await getDBType();
+  if (dbType === 'mysql5.7') {
+    query.append(SQL`
+        AND JSON_CONTAINS(m.content, CAST(up.id AS JSON), '$')
+    `);
+  } else {
+    query.append(SQL`
+        AND JSON_CONTAINS(m.content, up.id, '$')
+    `);
+  }
+  query.append(SQL`
     LEFT JOIN memberships mm ON mm.thread = m.thread AND mm.user = ${viewerID}
     LEFT JOIN memberships stm ON m.type = ${messageTypes.CREATE_SUB_THREAD}
       AND stm.thread = m.content AND stm.user = ${viewerID}
     WHERE JSON_EXTRACT(mm.permissions, ${visibleExtractString}) IS TRUE AND
-  `;
+  `);
   query.append(selectionClause);
   query.append(SQL`
     ORDER BY m.thread, m.time DESC
@@ -612,13 +645,24 @@
     FROM messages m
     LEFT JOIN uploads up
       ON m.type IN (${[messageTypes.IMAGES, messageTypes.MULTIMEDIA]})
-        AND JSON_CONTAINS(m.content, CAST(up.id as JSON), '$')
+  `;
+  const dbType = await getDBType();
+  if (dbType === 'mysql5.7') {
+    query.append(SQL`
+        AND JSON_CONTAINS(m.content, CAST(up.id AS JSON), '$')
+    `);
+  } else {
+    query.append(SQL`
+        AND JSON_CONTAINS(m.content, up.id, '$')
+    `);
+  }
+  query.append(SQL`
     LEFT JOIN memberships mm ON mm.thread = m.thread AND mm.user = ${viewerID}
     LEFT JOIN memberships stm ON m.type = ${messageTypes.CREATE_SUB_THREAD}
       AND stm.thread = m.content AND stm.user = ${viewerID}
     WHERE m.user = ${viewerID} AND m.creation = ${creation} AND
       JSON_EXTRACT(mm.permissions, ${visibleExtractString}) IS TRUE
-  `;
+  `);
 
   const [result] = await dbQuery(query);
   if (result.length === 0) {
@@ -643,13 +687,24 @@
     FROM messages m
     LEFT JOIN uploads up
       ON m.type IN (${[messageTypes.IMAGES, messageTypes.MULTIMEDIA]})
-        AND JSON_CONTAINS(m.content, CAST(up.id as JSON), '$')
+  `;
+  const dbType = await getDBType();
+  if (dbType === 'mysql5.7') {
+    query.append(SQL`
+        AND JSON_CONTAINS(m.content, CAST(up.id AS JSON), '$')
+    `);
+  } else {
+    query.append(SQL`
+        AND JSON_CONTAINS(m.content, up.id, '$')
+    `);
+  }
+  query.append(SQL`
     LEFT JOIN memberships mm ON mm.thread = m.thread AND mm.user = ${viewerID}
     WHERE m.user = ${viewerID} AND m.thread = ${threadID} AND
       m.type = ${messageType} AND
       JSON_EXTRACT(m.content, ${entryIDExtractString}) = ${entryID} AND
       JSON_EXTRACT(mm.permissions, ${visibleExtractString}) IS TRUE
-  `;
+  `);
 
   const [result] = await dbQuery(query);
   if (result.length === 0) {
@@ -668,11 +723,22 @@
     FROM messages m
     LEFT JOIN uploads up
       ON m.type IN (${[messageTypes.IMAGES, messageTypes.MULTIMEDIA]})
-        AND JSON_CONTAINS(m.content, CAST(up.id as JSON), '$')
+  `;
+  const dbType = await getDBType();
+  if (dbType === 'mysql5.7') {
+    query.append(SQL`
+        AND JSON_CONTAINS(m.content, CAST(up.id AS JSON), '$')
+    `);
+  } else {
+    query.append(SQL`
+        AND JSON_CONTAINS(m.content, up.id, '$')
+    `);
+  }
+  query.append(SQL`
     LEFT JOIN memberships stm ON m.type = ${messageTypes.CREATE_SUB_THREAD}
       AND stm.thread = m.content AND stm.user = m.user
     WHERE m.id IN (${messageIDs})
-  `;
+  `);
   const [result] = await dbQuery(query);
   return result;
 }