Subtle SQL differences: Constraint names

The various SQL product vendors implement subtle differences in the way they interpret SQL. In this case, I’ve been examining the reuse of constraint names within a schema / database (which is yet another story: what’s a schema, what’s a database?). Here’s the summary:

Constraint names are unique within a schema

  • Derby
  • H2
  • HSQLDB
  • Ingres
  • MySQL
  • Oracle
  • SQL Server

Constraint names are unique within a table

  • DB2
  • SQLite
  • Sybase SQL Anywhere

The “weird ones”

  • Postgres: Foreign Key names can be reused. Unique / Primary Key names cannot
  • Sybase ASE: Unique / Primary Key names can be reused. Foreign Key names cannot

For most compatibility across databases, it is never a good idea to re-use names. Keep your constraint names unique across a schema.

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