68 lines
2.1 KiB
Markdown
68 lines
2.1 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: The WHERE clause and compound statements
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Within the `SELECT` statement, the `WHERE` clause specifies the search criterion. The `WHERE` clause should always be last in the syntax. The clauses are always written in this order: `FROM` followed by `WHERE`.
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`SELECT name, cores, release_date FROM model WHERE CORES="4";`:
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```
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name cores release_date
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---------------------- ---------- ------------
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Raspberry Pi 2 Model B 4 2015-02-01
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Raspberry Pi 3 Model B 4 2016-02-01
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Raspberry Pi 3 Model B 4 2018-03-14
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Raspberry Pi 3 Model A 4 2018-11-01
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Raspberry Pi 4 Model B 4 2019-06-24
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Raspberry Pi 4 Model B 4 2019-06-24
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Raspberry Pi 4 Model B 4 2019-06-24
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Apple iMac 4 2019-03-19
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```
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## Compound statements
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Compound statements allow you to apply more filters to your clauses within an SQL statement. SQL allows you to build complex, combinatorial `WHERE` clauses by using Boolean and mathematical operators (i.e `AND` , `OR` , `>` , `<` , `!=` , `<=` ...)
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Multiple clauses:
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```sql
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SELECT name, ram, release_date
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FROM model
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WHERE release_date > '2018-01-01' AND ram > 512;
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```
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More complex logic achieve with parentheses:
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```sql
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SELECT name, cores, release_date
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FROM model
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WHERE (manufacturer_id = 1 OR manufacturer_id = 2) AND cores >= 2;
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```
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## Wildcards
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SQL does not use Regex. Instead it has a simpler glob-like syntax for carrying out string matching.
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In order to signal that you wish to compare by a wildcard and not a value, you have to use the `LIKE` keyword. The actual wildcard operator is `%` .
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In an SQL statement, the `%` wild card will match any number of occurrences of any character.
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Any characters can appear before or after ‘MacBook’ and the record will still be returned:
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```sql
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SELECT name, cores, release_date
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FROM model
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WHERE name LIKE '%MacBook%';
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```
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This wildcard only filters characters that come after `Raspberry` :
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```sql
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SELECT name, cores, release_date
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FROM model
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WHERE name LIKE 'Raspberry%';
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```
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