官术网_书友最值得收藏!

Selecting data

We now know how to retrieve data from the database, but that isn't much good unless we can get it back again. Let's say we now want to build an account settings page for MyStatus; we've got the user's username stored in a persistent session, but we will retrieve the other profile fields from the database to display in the settings form:

    SELECT * FROM "users"
WHERE "username" = 'alice';

This query tells Cassandra we want to retrieve the rows where the value for username (the primary key) is alice. The * wildcard simply says we would like all the columns in that row, saving us from having to type them all out. You'll see the rows we requested nicely formatted in the CQL shell as follows:

In other scenarios, we don't need all the columns. When a user tries to log in to MyStatus, we want to retrieve their password and compare it to the one the user provided us with, but we don't care about the email. Avoiding unnecessary columns reduces the amount of data that needs to be transferred between Cassandra and our application, thus making the queries faster. Instead of using the (*) wildcard, we can instead type a list of columns we are interested in:

    SELECT "username", "encrypted_password" FROM "users"
WHERE "username" = 'alice';

You'll see that in the results, the email column no longer appears since it wasn't in the list of columns that we specified.

主站蜘蛛池模板: 砀山县| 开化县| 炎陵县| 广丰县| 辽中县| 红安县| 保德县| 南安市| 昌邑市| 馆陶县| 凌云县| 泽州县| 岗巴县| 建始县| 萨迦县| 霸州市| 龙川县| 岑溪市| 宾川县| 长武县| 邮箱| 威海市| 泸水县| 丁青县| 海兴县| 集贤县| 徐州市| 盐山县| 班戈县| 邵阳市| 汉寿县| 垦利县| 察哈| 晋城| 湄潭县| 舞阳县| 天长市| 冀州市| 枣强县| 凯里市| 临沭县|