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Using SO as an example, what is the most sensible way to manage tags if you anticipate they will change often?

Way 1: Seriously denormalized (comma delimited)

table posts
+--------+-----------------+ 
| postId | tags            |
+--------+-----------------+
|   1    | c++,search,code |

Here tags are comma delimited.

Pros: Tags are retrieved at once with a single select query. Updating tags is simple. Easy and cheap to update.

Cons: Extra parsing on tag retrieval, difficult to count how many posts use which tags.

(alternatively, if limited to something like 5 tags)

table posts
+--------+-------+-------+-------+-------+-------+
| postId | tag_1 | tag_2 | tag_3 | tag_4 | tag_5 |
+--------+-------+-------+-------+-------+-------+
|   1    | c++   |search | code  |       |       | 

Way 2: "Slightly normalized" (separate table, no intersection)

table posts
+--------+-------------------+
| postId | title             |
+--------+-------------------+
|   1    | How do u tag?     |

table taggings
+--------+---------+
| postId | tagName |
+--------+---------+
|   1    | C++     |
|   1    | search  |

Pros: Easy to see tag counts (count(*) from taggings where tagName='C++').

Cons: tagName will likely be repeated many, many times.

Way 3: The cool kid’s (normalized with intersection table)

table posts
+--------+---------------------------------------+
| postId | title                                 |
+--------+---------------------------------------+
|   1    | Why is a raven like a writing desk?   |

table tags
+--------+---------+
| tagId  | tagName |
+--------+---------+
|   1    | C++     |
|   2    | search  |
|   3    | foofle  |

table taggings
+--------+---------+
| postId | tagId   |
+--------+---------+
|   1    | 1       |
|   1    | 2       |
|   1    | 3       |

Pros:

• No repeating tag names.
• More girls will like you.

Cons: More expensive to change tags than way #1.

Answer 1

These solutions are called mysqlicious, scuttle and toxi.

This article compares benefits and drawbacks of each.