51 lines
1.9 KiB
Markdown
51 lines
1.9 KiB
Markdown
---
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categories:
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- Databases
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- Programming Languages
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tags: [SQL]
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---
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# SQL: Language structure
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Before we start using the syntax we need to understand the grammar:
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Expressions differ from clauses and predicates in that they are not the mechanism for returning data (i.e. declaring a clause and a logical colllllndition) they do something to the data, as part of the retrieval. This is a bit subtle:
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- `SELECT name FROM model WHERE cores = "4"`
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- This retrieves the models that have 4 cores
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- `SELECT count(*) FROM model WHERE cores = "4" `
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- This counts the number of models that are returned where the counting is a function over and above the retrieval itself.
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### Examples from `computer_sales.db`
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`sqlite> SELECT * from model WHERE cpu_speed=0.7` : return all models with a CPU speed equal to 0.7:
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```
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model_id manufacturer_id name cpu_speed ram cores wifi release_date
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---------- --------------- ---------------------- ---------- ---------- ---------- ---------- ------------
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1 1 Raspberry Pi 1 Model A 0.7 256.0 1 0 2013-02-01
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2 1 Raspberry Pi 1 Model B 0.7 256.0 1 0 2012-04-01
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3 1 Raspberry Pi 1 Model B 0.7 512.0 1 0 2012-10-01
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4 1 Raspberry Pi 1 Model A 0.7 512.0 1 0 2014-11-01
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5 1 Raspberry Pi 1 Model B 0.7 512.0 1 0 2014-07-01
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```
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```
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count(*)
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----------
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5
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```
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> Any value that is not a number should be in single-quotes, never double quotes
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## Main commands
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There are obviously many SQL commands but most standard CRUD actions can be executed with a small number of commands:
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- `SELECT`
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- `UPDATE`
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- `CREATE`
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- `INSERT`
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- `DELETE`
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