Condition (SQL)

A relational database management system uses SQL conditions or expressions in WHERE clauses and in HAVING clauses to SELECT subsets of data.

Types of condition
  • Many conditions compare values for (for example) equality, inequality or similarity.
  • The EXISTS condition uses the SQL standard keyword EXISTS[1] to determine whether rows exist in a subquery result.[2]

Examples

To SELECT one row of data from a table called tab with a primary key column (pk) set to 100 — use the condition pk = 100:

SELECT * FROM tab WHERE pk = 100

To identify whether a table tab has rows of data with a duplicated column dk — use the condition having count(*) > 1:

SELECT dk FROM tab GROUP BY dk HAVING count(*) > 1
gollark: There's also a trigram tokenizer which lets you do LIKE very fast.
gollark: Yes.
gollark: SQLite also stores the position of tokens and has a stats table tracking frequency in all documents and such for better ranking.
gollark: It uses porter stemming to normalize "bee" and "bees" into "bee".
gollark: Anyway, SQLite documents it, but the basic idea is that you have a table like this```sqlCREATE TABLE thing ( token TEXT NOT NULL, document_id INTEGER NOT NULL);```so when you look up "bees" it searches for the token "bees" in there using an index and retrieves the resulting document.

References
  1. Fehily, Chris (2005). SQL: Visual Quickstart Guide (2 ed.). Peachpit Press. pp. 439–440, 480. ISBN 978-0-321-33417-6. SQL Keywords [...] The appendix lists the SQL:2003 standard's reserved and non-reserved keywords. [...] EXISTS [...]
  2. Fehily, Chris (2005). SQL: Visual Quickstart Guide (2 ed.). Peachpit Press. p. 278. ISBN 978-0-321-33417-6. EXISTS and NOT EXISTS [...] look for the existence or nonexistence of rows in a subquery result.
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