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Posted to commits@cassandra.apache.org by sl...@apache.org on 2013/09/05 16:38:29 UTC
svn commit: r1520330 [1/2] - in /cassandra/site/publish/doc/cql3:
CQL-1.2.html CQL-2.0.html
Author: slebresne
Date: Thu Sep 5 14:38:29 2013
New Revision: 1520330
URL: http://svn.apache.org/r1520330
Log:
CQL doc update
Modified:
cassandra/site/publish/doc/cql3/CQL-1.2.html
cassandra/site/publish/doc/cql3/CQL-2.0.html
Modified: cassandra/site/publish/doc/cql3/CQL-1.2.html
URL: http://svn.apache.org/viewvc/cassandra/site/publish/doc/cql3/CQL-1.2.html?rev=1520330&r1=1520329&r2=1520330&view=diff
==============================================================================
--- cassandra/site/publish/doc/cql3/CQL-1.2.html (original)
+++ cassandra/site/publish/doc/cql3/CQL-1.2.html Thu Sep 5 14:38:29 2013
@@ -93,7 +93,7 @@ CREATE TABLE timeline (
other text,
PRIMARY KEY (k)
)
-</pre></pre><p>Moreover, a table must define at least one column that is not part of the PRIMARY KEY as a row exists in Cassandra only if it contains at least one value for one such column.</p><h4 id="createTablepartitionClustering">Partition key and clustering</h4><p>In CQL, the order in which columns are defined for the <code>PRIMARY KEY</code> matters. The first column of the key is called the <i>partition key</i>. It has the property that all the rows sharing the same partition key (even across table in fact) are stored on the same physical node. Also, insertion/update/deletion on rows sharing the same partition key for a given table are performed <i>atomically</i> and in <i>isolation</i>. Note that it is possible to have a composite partition key, i.e. a partition key formed of multiple columns, using an extra set of parentheses to define which columns forms the partition key.</p><p>The remaining columns of the <code>PRIMARY KEY</code> definition, if any, are called <i>clusteri
ng keys</i>. On a given physical node, rows for a given partition key are stored in the order induced by the clustering keys, making the retrieval of rows in that clustering order particularly efficient (see <a href="#selectStmt"><tt>SELECT</tt></a>).</p><h4 id="createTableOptions"><code><option></code></h4><p>The <code>CREATE TABLE</code> statement supports a number of options that controls the configuration of a new table. These options can be specified after the <code>WITH</code> keyword.</p><p>The first of these option is <code>COMPACT STORAGE</code>. This option is meanly targeted towards backward compatibility with some table definition created before CQL3. But it also provides a slightly more compact layout of data on disk, though at the price of flexibility and extensibility, and for that reason is not recommended unless for the backward compatibility reason. The restriction for table with <code>COMPACT STORAGE</code> is that they support one and only one column outside
of the ones part of the <code>PRIMARY KEY</code>. It also follows that columns cannot be added nor removed after creation. A table with <code>COMPACT STORAGE</code> must also define at least one <a href="createTablepartitionClustering">clustering key</a>.</p><p>Another option is <code>CLUSTERING ORDER</code>. It allows to define the ordering of rows on disk. It takes the list of the clustering key names with, for each of them, the on-disk order (Ascending or descending). Note that this option affects <a href="#selectOrderBy">what <code>ORDER BY</code> are allowed during <code>SELECT</code></a>.</p><p>Table creation supports the following other <code><property></code>:</p><table><tr><th>option </th><th>kind </th><th>default </th><th>description</th></tr><tr><td><code>comment</code> </td><td><em>simple</em> </td><td>none </td><td>A free-form, human-readable comment.</td></tr><tr><td><code>read_repair_chance</code> </td><td><e
m>simple</em> </td><td>0.1 </td><td>The probability with which to query extra nodes (e.g. more nodes than required by the consistency level) for the purpose of read repairs.</td></tr><tr><td><code>dclocal_read_repair_chance</code> </td><td><em>simple</em> </td><td>0 </td><td>The probability with which to query extra nodes (e.g. more nodes than required by the consistency level) belonging to the same data center than the read coordinator for the purpose of read repairs.</td></tr><tr><td><code>gc_grace_seconds</code> </td><td><em>simple</em> </td><td>864000 </td><td>Time to wait before garbage collecting tombstones (deletion markers).</td></tr><tr><td><code>bloom_filter_fp_chance</code> </td><td><em>simple</em> </td><td>0.00075 </td><td>The target probability of false positive of the sstable bloom filters. Said bloom filters will be sized to provide the provided probability (thus lowering this value impact the size of bloom filters in-memory an
d on-disk)</td></tr><tr><td><code>compaction</code> </td><td><em>map</em> </td><td><em>see below</em> </td><td>The compaction otpions to use, see below.</td></tr><tr><td><code>compression</code> </td><td><em>map</em> </td><td><em>see below</em> </td><td>Compression options, see below. </td></tr><tr><td><code>replicate_on_write</code> </td><td><em>simple</em> </td><td>true </td><td>Whether to replicate data on write. This can only be set to false for tables with counters values. Disabling this is dangerous and can result in random lose of counters, don’t disable unless you are sure to know what you are doing</td></tr><tr><td><code>caching</code> </td><td><em>simple</em> </td><td>keys_only </td><td>Whether to cache keys (“key cache”) and/or rows (“row cache”) for this table. Valid values are: <code>all</code>, <code>keys_only</code>, <code>rows_only</code> and <code>none</code>. </td
></tr></table><h4 id="compactionOptions"><code>compaction</code> options</h4><p>The <code>compaction</code> property must at least define the <code>'class'</code> sub-option, that defines the compaction strategy class to use. The default supported class are <code>'SizeTieredCompactionStrategy'</code> and <code>'LeveledCompactionStrategy'</code>. Custom strategy can be provided by specifying the full class name as a <a href="#constants">string constant</a>. The rest of the sub-options depends on the chosen class. The sub-options supported by the default classes are:</p><table><tr><th>option </th><th>supported compaction strategy </th><th>default </th><th>description </th></tr><tr><td><code>tombstone_threshold</code> </td><td><em>all</em> </td><td>0.2 </td><td>A ratio such that if a sstable has more than this ratio of gcable tombstones over all contained columns, the sstable will be compacted (with no other sstables) for
the purpose of purging those tombstones. </td></tr><tr><td><code>tombstone_compaction_interval</code> </td><td><em>all</em> </td><td>1 day </td><td>The mininum time to wait after an sstable creation time before considering it for “tombstone compaction”, where “tombstone compaction” is the compaction triggered if the sstable has more gcable tombstones than <code>tombstone_threshold</code>. </td></tr><tr><td><code>min_sstable_size</code> </td><td>SizeTieredCompactionStrategy </td><td>50MB </td><td>The size tiered strategy groups SSTables to compact in buckets. A bucket groups SSTables that differs from less than 50% in size. However, for small sizes, this would result in a bucketing that is too fine grained. <code>min_sstable_size</code> defines a size threshold (in bytes) below which all SSTables belong to one unique bucket</td></tr><tr><td><code>min_threshold</code> </td><td>SizeTieredCompac
tionStrategy </td><td>4 </td><td>Minimum number of SSTables needed to start a minor compaction.</td></tr><tr><td><code>max_threshold</code> </td><td>SizeTieredCompactionStrategy </td><td>32 </td><td>Maximum number of SSTables processed by one minor compaction.</td></tr><tr><td><code>bucket_low</code> </td><td>SizeTieredCompactionStrategy </td><td>0.5 </td><td>Size tiered consider sstables to be within the same bucket if their size is within [average_size * <code>bucket_low</code>, average_size * <code>bucket_high</code> ] (i.e the default groups sstable whose sizes diverges by at most 50%)</td></tr><tr><td><code>bucket_high</code> </td><td>SizeTieredCompactionStrategy </td><td>1.5 </td><td>Size tiered consider sstables to be within the same bucket if their size is within [average_size * <code>bucket_low</code>, average_size * <code>bucket_high</code> ] (i.e the default groups sstable whose si
zes diverges by at most 50%).</td></tr><tr><td><code>sstable_size_in_mb</code> </td><td>LeveledCompactionStrategy </td><td>5MB </td><td>The target size (in MB) for sstables in the leveled strategy. Note that while sstable sizes should stay less or equal to <code>sstable_size_in_mb</code>, it is possible to exceptionally have a larger sstable as during compaction, data for a given partition key are never split into 2 sstables</td></tr></table><p>For the <code>compression</code> property, the following default sub-options are available:</p><table><tr><th>option </th><th>default </th><th>description </th></tr><tr><td><code>sstable_compression</code> </td><td>SnappyCompressor </td><td>The compression algorithm to use. Default compressor are: SnappyCompressor and DeflateCompressor. Use an empty string (<code>''</code>) to disable compression. Custom compressor can be provided by specifying the full class name as a <a href="#constants">string con
stant</a>.</td></tr><tr><td><code>chunk_length_kb</code> </td><td>64KB </td><td>On disk SSTables are compressed by block (to allow random reads). This defines the size (in KB) of said block. Bigger values may improve the compression rate, but increases the minimum size of data to be read from disk for a read </td></tr><tr><td><code>crc_check_chance</code> </td><td>1.0 </td><td>When compression is enabled, each compressed block includes a checksum of that block for the purpose of detecting disk bitrot and avoiding the propagation of corruption to other replica. This option defines the probability with which those checksums are checked during read. By default they are always checked. Set to 0 to disable checksum checking and to 0.5 for instance to check them every other read</td></tr></table><h4 id="Otherconsiderations">Other considerations:</h4><ul><li>When <a href="#insertStmt/"updating":#updateStmt">inserting</a> a given row, not all column
s needs to be defined (except for those part of the key), and missing columns occupy no space on disk. Furthermore, adding new columns (see <a href=#alterStmt><tt>ALTER TABLE</tt></a>) is a constant time operation. There is thus no need to try to anticipate future usage (or to cry when you haven’t) when creating a table.</li></ul><h3 id="alterTableStmt">ALTER TABLE</h3><p><i>Syntax:</i></p><pre class="syntax"><pre><alter-table-stmt> ::= ALTER (TABLE | COLUMNFAMILY) <tablename> <instruction>
+</pre></pre><p>Moreover, a table must define at least one column that is not part of the PRIMARY KEY as a row exists in Cassandra only if it contains at least one value for one such column.</p><h4 id="createTablepartitionClustering">Partition key and clustering</h4><p>In CQL, the order in which columns are defined for the <code>PRIMARY KEY</code> matters. The first column of the key is called the <i>partition key</i>. It has the property that all the rows sharing the same partition key (even across table in fact) are stored on the same physical node. Also, insertion/update/deletion on rows sharing the same partition key for a given table are performed <i>atomically</i> and in <i>isolation</i>. Note that it is possible to have a composite partition key, i.e. a partition key formed of multiple columns, using an extra set of parentheses to define which columns forms the partition key.</p><p>The remaining columns of the <code>PRIMARY KEY</code> definition, if any, are called __clusterin
g columns. On a given physical node, rows for a given partition key are stored in the order induced by the clustering columns, making the retrieval of rows in that clustering order particularly efficient (see <a href="#selectStmt"><tt>SELECT</tt></a>).</p><h4 id="createTableOptions"><code><option></code></h4><p>The <code>CREATE TABLE</code> statement supports a number of options that controls the configuration of a new table. These options can be specified after the <code>WITH</code> keyword.</p><p>The first of these option is <code>COMPACT STORAGE</code>. This option is meanly targeted towards backward compatibility with some table definition created before CQL3. But it also provides a slightly more compact layout of data on disk, though at the price of flexibility and extensibility, and for that reason is not recommended unless for the backward compatibility reason. The restriction for table with <code>COMPACT STORAGE</code> is that they support one and only one column outside
of the ones part of the <code>PRIMARY KEY</code>. It also follows that columns cannot be added nor removed after creation. A table with <code>COMPACT STORAGE</code> must also define at least one <a href="createTablepartitionClustering">clustering key</a>.</p><p>Another option is <code>CLUSTERING ORDER</code>. It allows to define the ordering of rows on disk. It takes the list of the clustering key names with, for each of them, the on-disk order (Ascending or descending). Note that this option affects <a href="#selectOrderBy">what <code>ORDER BY</code> are allowed during <code>SELECT</code></a>.</p><p>Table creation supports the following other <code><property></code>:</p><table><tr><th>option </th><th>kind </th><th>default </th><th>description</th></tr><tr><td><code>comment</code> </td><td><em>simple</em> </td><td>none </td><td>A free-form, human-readable comment.</td></tr><tr><td><code>read_repair_chance</code> </td><td><
em>simple</em> </td><td>0.1 </td><td>The probability with which to query extra nodes (e.g. more nodes than required by the consistency level) for the purpose of read repairs.</td></tr><tr><td><code>dclocal_read_repair_chance</code> </td><td><em>simple</em> </td><td>0 </td><td>The probability with which to query extra nodes (e.g. more nodes than required by the consistency level) belonging to the same data center than the read coordinator for the purpose of read repairs.</td></tr><tr><td><code>gc_grace_seconds</code> </td><td><em>simple</em> </td><td>864000 </td><td>Time to wait before garbage collecting tombstones (deletion markers).</td></tr><tr><td><code>bloom_filter_fp_chance</code> </td><td><em>simple</em> </td><td>0.00075 </td><td>The target probability of false positive of the sstable bloom filters. Said bloom filters will be sized to provide the provided probability (thus lowering this value impact the size of bloom filters in-memory a
nd on-disk)</td></tr><tr><td><code>compaction</code> </td><td><em>map</em> </td><td><em>see below</em> </td><td>The compaction otpions to use, see below.</td></tr><tr><td><code>compression</code> </td><td><em>map</em> </td><td><em>see below</em> </td><td>Compression options, see below. </td></tr><tr><td><code>replicate_on_write</code> </td><td><em>simple</em> </td><td>true </td><td>Whether to replicate data on write. This can only be set to false for tables with counters values. Disabling this is dangerous and can result in random lose of counters, don’t disable unless you are sure to know what you are doing</td></tr><tr><td><code>caching</code> </td><td><em>simple</em> </td><td>keys_only </td><td>Whether to cache keys (“key cache”) and/or rows (“row cache”) for this table. Valid values are: <code>all</code>, <code>keys_only</code>, <code>rows_only</code> and <code>none</code>. </t
d></tr></table><h4 id="compactionOptions"><code>compaction</code> options</h4><p>The <code>compaction</code> property must at least define the <code>'class'</code> sub-option, that defines the compaction strategy class to use. The default supported class are <code>'SizeTieredCompactionStrategy'</code> and <code>'LeveledCompactionStrategy'</code>. Custom strategy can be provided by specifying the full class name as a <a href="#constants">string constant</a>. The rest of the sub-options depends on the chosen class. The sub-options supported by the default classes are:</p><table><tr><th>option </th><th>supported compaction strategy </th><th>default </th><th>description </th></tr><tr><td><code>tombstone_threshold</code> </td><td><em>all</em> </td><td>0.2 </td><td>A ratio such that if a sstable has more than this ratio of gcable tombstones over all contained columns, the sstable will be compacted (with no other sstables) fo
r the purpose of purging those tombstones. </td></tr><tr><td><code>tombstone_compaction_interval</code> </td><td><em>all</em> </td><td>1 day </td><td>The mininum time to wait after an sstable creation time before considering it for “tombstone compaction”, where “tombstone compaction” is the compaction triggered if the sstable has more gcable tombstones than <code>tombstone_threshold</code>. </td></tr><tr><td><code>min_sstable_size</code> </td><td>SizeTieredCompactionStrategy </td><td>50MB </td><td>The size tiered strategy groups SSTables to compact in buckets. A bucket groups SSTables that differs from less than 50% in size. However, for small sizes, this would result in a bucketing that is too fine grained. <code>min_sstable_size</code> defines a size threshold (in bytes) below which all SSTables belong to one unique bucket</td></tr><tr><td><code>min_threshold</code> </td><td>SizeTieredCompa
ctionStrategy </td><td>4 </td><td>Minimum number of SSTables needed to start a minor compaction.</td></tr><tr><td><code>max_threshold</code> </td><td>SizeTieredCompactionStrategy </td><td>32 </td><td>Maximum number of SSTables processed by one minor compaction.</td></tr><tr><td><code>bucket_low</code> </td><td>SizeTieredCompactionStrategy </td><td>0.5 </td><td>Size tiered consider sstables to be within the same bucket if their size is within [average_size * <code>bucket_low</code>, average_size * <code>bucket_high</code> ] (i.e the default groups sstable whose sizes diverges by at most 50%)</td></tr><tr><td><code>bucket_high</code> </td><td>SizeTieredCompactionStrategy </td><td>1.5 </td><td>Size tiered consider sstables to be within the same bucket if their size is within [average_size * <code>bucket_low</code>, average_size * <code>bucket_high</code> ] (i.e the default groups sstable whose s
izes diverges by at most 50%).</td></tr><tr><td><code>sstable_size_in_mb</code> </td><td>LeveledCompactionStrategy </td><td>5MB </td><td>The target size (in MB) for sstables in the leveled strategy. Note that while sstable sizes should stay less or equal to <code>sstable_size_in_mb</code>, it is possible to exceptionally have a larger sstable as during compaction, data for a given partition key are never split into 2 sstables</td></tr></table><p>For the <code>compression</code> property, the following default sub-options are available:</p><table><tr><th>option </th><th>default </th><th>description </th></tr><tr><td><code>sstable_compression</code> </td><td>SnappyCompressor </td><td>The compression algorithm to use. Default compressor are: SnappyCompressor and DeflateCompressor. Use an empty string (<code>''</code>) to disable compression. Custom compressor can be provided by specifying the full class name as a <a href="#constants">string co
nstant</a>.</td></tr><tr><td><code>chunk_length_kb</code> </td><td>64KB </td><td>On disk SSTables are compressed by block (to allow random reads). This defines the size (in KB) of said block. Bigger values may improve the compression rate, but increases the minimum size of data to be read from disk for a read </td></tr><tr><td><code>crc_check_chance</code> </td><td>1.0 </td><td>When compression is enabled, each compressed block includes a checksum of that block for the purpose of detecting disk bitrot and avoiding the propagation of corruption to other replica. This option defines the probability with which those checksums are checked during read. By default they are always checked. Set to 0 to disable checksum checking and to 0.5 for instance to check them every other read</td></tr></table><h4 id="Otherconsiderations">Other considerations:</h4><ul><li>When <a href="#insertStmt/"updating":#updateStmt">inserting</a> a given row, not all colum
ns needs to be defined (except for those part of the key), and missing columns occupy no space on disk. Furthermore, adding new columns (see <a href=#alterStmt><tt>ALTER TABLE</tt></a>) is a constant time operation. There is thus no need to try to anticipate future usage (or to cry when you haven’t) when creating a table.</li></ul><h3 id="alterTableStmt">ALTER TABLE</h3><p><i>Syntax:</i></p><pre class="syntax"><pre><alter-table-stmt> ::= ALTER (TABLE | COLUMNFAMILY) <tablename> <instruction>
<instruction> ::= ALTER <identifier> TYPE <type>
| ADD <identifier> <type>
@@ -107,7 +107,7 @@ ADD gravesite varchar;
ALTER TABLE addamsFamily
WITH comment = 'A most excellent and useful column family'
AND read_repair_chance = 0.2;
-</pre></pre><p><br/>The <code>ALTER</code> statement is used to manipulate table definitions. It allows to add new columns, drop existing ones, change the type of existing columns, or update the table options. As for table creation, <code>ALTER COLUMNFAMILY</code> is allowed as an alias for <code>ALTER TABLE</code>.</p><p>The <code><tablename></code> is the table name optionally preceded by the keyspace name. The <code><instruction></code> defines the alteration to perform:</p><ul><li><code>ALTER</code>: Update the type of a given defined column. Note that the type of the <a href="#createTablepartitionClustering">clustering keys</a> cannot be modified as it induces the on-disk ordering of rows. Columns on which a <a href="#createIndexStmt">secondary index</a> is defined have the same restriction. Other columns are free from those restrictions (no validation of existing data is performed), but it is usually a bad idea to change the type to a non-compatible one, unless no data
have been inserted for that column yet, as this could confuse CQL drivers/tools.</li><li><code>ADD</code>: Adds a new column to the table. The <code><identifier></code> for the new column must not conflict with an existing column. Moreover, columns cannot be added to tables defined with the <code>COMPACT STORAGE</code> option.</li><li><code>WITH</code>: Allows to update the options of the table. The <a href="#createTableOptions">supported <code><option></code></a> (and syntax) are the same as for the <code>CREATE TABLE</code> statement except that <code>COMPACT STORAGE</code> is not supported. Note that setting any <code>compaction</code> sub-options has the effect of erasing all previous <code>compaction</code> options, so you need to re-specify all the sub-options if you want to keep them. The same note applies to the set of <code>compression</code> sub-options.</li></ul><p>Dropping a column is no yet supported but is on <a href="https://issues.apache.org/jira/browse/CASSAN
DRA-3919">the roadmap</a>. In the meantime, a declared but unused column has no impact on performance nor uses any storage.</p><h3 id="dropTableStmt">DROP TABLE</h3><p><i>Syntax:</i></p><pre class="syntax"><pre><drop-table-stmt> ::= DROP TABLE <tablename>
+</pre></pre><p><br/>The <code>ALTER</code> statement is used to manipulate table definitions. It allows to add new columns, drop existing ones, change the type of existing columns, or update the table options. As for table creation, <code>ALTER COLUMNFAMILY</code> is allowed as an alias for <code>ALTER TABLE</code>.</p><p>The <code><tablename></code> is the table name optionally preceded by the keyspace name. The <code><instruction></code> defines the alteration to perform:</p><ul><li><code>ALTER</code>: Update the type of a given defined column. Note that the type of the <a href="#createTablepartitionClustering">clustering columns</a> cannot be modified as it induces the on-disk ordering of rows. Columns on which a <a href="#createIndexStmt">secondary index</a> is defined have the same restriction. Other columns are free from those restrictions (no validation of existing data is performed), but it is usually a bad idea to change the type to a non-compatible one, unless no da
ta have been inserted for that column yet, as this could confuse CQL drivers/tools.</li><li><code>ADD</code>: Adds a new column to the table. The <code><identifier></code> for the new column must not conflict with an existing column. Moreover, columns cannot be added to tables defined with the <code>COMPACT STORAGE</code> option.</li><li><code>WITH</code>: Allows to update the options of the table. The <a href="#createTableOptions">supported <code><option></code></a> (and syntax) are the same as for the <code>CREATE TABLE</code> statement except that <code>COMPACT STORAGE</code> is not supported. Note that setting any <code>compaction</code> sub-options has the effect of erasing all previous <code>compaction</code> options, so you need to re-specify all the sub-options if you want to keep them. The same note applies to the set of <code>compression</code> sub-options.</li></ul><p>Dropping a column is no yet supported but is on <a href="https://issues.apache.org/jira/browse/CAS
SANDRA-3919">the roadmap</a>. In the meantime, a declared but unused column has no impact on performance nor uses any storage.</p><h3 id="dropTableStmt">DROP TABLE</h3><p><i>Syntax:</i></p><pre class="syntax"><pre><drop-table-stmt> ::= DROP TABLE <tablename>
</pre></pre><p><i>Sample:</i></p><pre class="sample"><pre>DROP TABLE worldSeriesAttendees;
</pre></pre><p>The <code>DROP TABLE</code> statement results in the immediate, irreversible removal of a table, including all data contained in it. As for table creation, <code>DROP COLUMNFAMILY</code> is allowed as an alias for <code>DROP TABLE</code>.</p><h3 id="truncateStmt">TRUNCATE</h3><p><i>Syntax:</i></p><pre class="syntax"><pre><truncate-stmt> ::= TRUNCATE <tablename>
</pre></pre><p><i>Sample:</i></p><pre class="sample"><pre>TRUNCATE superImportantData;
@@ -220,7 +220,7 @@ WHERE event_type = 'myEvent'
AND time <= '2012-01-01'
SELECT COUNT(*) FROM users;
-</pre></pre><p><br/>The <code>SELECT</code> statements reads one or more columns for one or more rows in a table. It returns a result-set of rows, where each row contains the collection of columns corresponding to the query.</p><h4 id="selectSelection"><code><select-clause></code></h4><p>The <code><select-clause></code> determines which columns needs to be queried and returned in the result-set. It consists of either the comma-separated list of <selector> or the wildcard character (<code>*</code>) to select all the columns defined for the table.</p><p>A <code><selector></code> is either a column name to retrieve, or a <code><function></code> of one or multiple column names. The functions allows are the same that for <code><term></code> and are describe in the <a href="#function">function section</a>. In addition to these generic functions, the <code>WRITETIME</code> (resp. <code>TTL</code>) function allows to select the timestamp of when the column was inserted (resp.
the time to live (in seconds) for the column (or null if the column has no expiration set)).</p><p>The <code>COUNT</code> keyword can be used with parenthesis enclosing <code>*</code>. If so, the query will return a single result: the number of rows matching the query. Note that <code>COUNT(1)</code> is supported as an alias.</p><h4 id="selectWhere"><code><where-clause></code></h4><p>The <code><where-clause></code> specifies which rows must be queried. It is composed of relations on the columns that are part of the <code>PRIMARY KEY</code> and/or have a <a href="#createIndexStmt">secondary index</a> defined on them.</p><p>Not all relations are allowed in a query. For instance, non-equal relations (where <code>IN</code> is considered as an equal relation) on a partition key are not supported (but see the use of the <code>TOKEN</code> method below to do non-equal queries on the partition key). Moreover, for a given partition key, the clustering keys induce an ordering of rows a
nd relations on them is restricted to the relations that allow to select a <strong>contiguous</strong> (for the ordering) set of rows. For instance, given</p><pre class="sample"><pre>CREATE TABLE posts (
+</pre></pre><p><br/>The <code>SELECT</code> statements reads one or more columns for one or more rows in a table. It returns a result-set of rows, where each row contains the collection of columns corresponding to the query.</p><h4 id="selectSelection"><code><select-clause></code></h4><p>The <code><select-clause></code> determines which columns needs to be queried and returned in the result-set. It consists of either the comma-separated list of <selector> or the wildcard character (<code>*</code>) to select all the columns defined for the table.</p><p>A <code><selector></code> is either a column name to retrieve, or a <code><function></code> of one or multiple column names. The functions allows are the same that for <code><term></code> and are describe in the <a href="#function">function section</a>. In addition to these generic functions, the <code>WRITETIME</code> (resp. <code>TTL</code>) function allows to select the timestamp of when the column was inserted (resp.
the time to live (in seconds) for the column (or null if the column has no expiration set)).</p><p>The <code>COUNT</code> keyword can be used with parenthesis enclosing <code>*</code>. If so, the query will return a single result: the number of rows matching the query. Note that <code>COUNT(1)</code> is supported as an alias.</p><h4 id="selectWhere"><code><where-clause></code></h4><p>The <code><where-clause></code> specifies which rows must be queried. It is composed of relations on the columns that are part of the <code>PRIMARY KEY</code> and/or have a <a href="#createIndexStmt">secondary index</a> defined on them.</p><p>Not all relations are allowed in a query. For instance, non-equal relations (where <code>IN</code> is considered as an equal relation) on a partition key are not supported (but see the use of the <code>TOKEN</code> method below to do non-equal queries on the partition key). Moreover, for a given partition key, the clustering columns induce an ordering of row
s and relations on them is restricted to the relations that allow to select a <strong>contiguous</strong> (for the ordering) set of rows. For instance, given</p><pre class="sample"><pre>CREATE TABLE posts (
userid text,
blog_title text,
posted_at timestamp,