What's the difference between Google Cloud Spanner and Cloud SQL?

There is not a big difference between then in terms on what they do (storing data in tables). The difference is how they handle the data in a small and big scale

Cloud Spanner is used when you need to handle massive amounts of data with an elevated level of consistency and with a big amount of data handling (+100,000 reads/write per second). Spanner gives much better scalability and better SLOs.

On the other hand, Spanner is also much more expensive than Cloud SQL.

If you just want to store some data of your customer in a cheap way but still don't want to face server configuration Cloud SQL is the right choice.

If you are planning to create a big product or if you want to be ready for a huge increase in users for your application (viral games/applications) Spanner is the right product.

You can find detailed information about Cloud Spanner in this official paper

The main difference between cloud spanner and cloud SQL is the horizontal scalability + globally available of data over 10TB.

Spanner isn’t for generic SQL needs, Spanner is best used for massive-scale opportunities. 1000s of writes per second, globally. 10,000s - 100,000s of reads per second, globally.

Above volume is extremely difficult to achieve with NORMAL SQL / MySQL without doing complex sharding of the database. Spanner deals with all this AND allows ACID updates (which is basically impossible with sharded databases). They accomplish this with super-accurate clocks to manage conflicts.

In short, SPanner is not for CRM database, it is more for supermassive global data within an organisation and since spanner is bit expensive (compared to cloud SQL) the project should be large enough to justify the cost of spanner.

You can also follow this discussion on Reddit (a good one!): https://www.reddit.com/r/googlecloud/comments/93bxf6/cloud_spanner_vs_cloud_sql/e3cof2r/